PHP RFC: Add RFC 3986 and WHATWG compliant URI parsing support

• Version: 1.0
• Date: 2024-06-11
• Author: Máté Kocsis, kocsismate@php.net
• Status: Under Discussion
• First Published at: https://wiki.php.net/rfc/url_parsing_api
• Implementation: https://github.com/php/php-src/pull/14461

URIs and URLs are one of the most fundamental concepts of the web because they make it possible to reference specific resources on a network. URLs were originally defined by Tim Berners-Lee in RFC 1738, but since then other specifications have also emerged, out of which RFC 3986 and WHATWG URL are the most notable ones. The former one updates the original RFC 1738 and defines URIs, while the latter one specifies how browsers should treat URLs.

Despite the ubiquitous nature of URLs and URIs, they are not so unequivocal as people may think, because different clients treat and parse them differently by either following one of the standards, or even worse, not following any at all. Unfortunately, PHP falls into the latter category: the parse_url() function is offered for parsing URLs, however, it isn't compliant with any standards. Even the PHP manual contains the following warning:

This function may not give correct results for relative or invalid URLs, and the results may not even match common behavior of HTTP clients. ...

Incompatibility with current standards is a serious issue, as it hinders interoperability with different tools (i.e. HTTP clients), or it can result in bugs which are difficult to notice. For example, cURL's URL parsing implementation is based on RFC 3986, that's why URLs validated by FILTER_VALIDATE_URL may not necessarily be accepted when passed to cURL.

In order to address the above mentioned problems, a new, always available URI extension is to be added to the standard library. The extension would support parsing, validating, modifying, and recomposing (converting the parsed structures back to strings) URIs based on both RFC 3986 and the WHATWG URL specifications, as well as resolving references (turning a (relative) URI to an absolute one by applying a base URI to it). For this purpose, the following internal classes and methods are added:

namespace Uri;
 
abstract class UriException extends \Exception
{
}
 
class UninitializedUriException extends \Uri\UriException
{
}
 
class UriOperationException extends \Uri\UriException
{
}
 
class InvalidUriException extends \Uri\UriException
{
    public readonly array $errors;
}
 
interface Uri
{
    public function getScheme(): ?string;
 
    public function withScheme(?string $scheme): static;
 
    public function getUser(): ?string;
 
    public function withUser(?string $user): static;
 
    public function getPassword(): ?string;
 
    public function withPassword(?string $password): static;
 
    public function getHost(): ?string;
 
    public function withHost(?string $host): static;
 
    public function getPort(): ?int;
 
    public function withPort(?int $port): static;
 
    public function getPath(): ?string;
 
    public function withPath(?string $path): static;
 
    public function getQuery(): ?string;
 
    public function withQuery(?string $query): static;
 
    public function getFragment(): ?string;
 
    public function withFragment(?string $fragment): static;
 
    public function equals(\Uri\Uri $uri, bool $excludeFragment = true): bool;
 
    public function normalize(): static;
 
    public function toNormalizedString(): string;
 
    public function toString(): string;
 
    public function resolve(string $uri): static;
}
 
readonly class Rfc3986Uri implements \Uri\Uri
{
    private ?string $scheme;
    private ?string $user;
    private ?string $password;
    private ?string $host;
    private ?int $port;
    private ?string $path;
    private ?string $query;
    private ?string $fragment;
 
    public static function parse(string $uri, ?string $baseUrl = null): ?static {}
 
    public function __construct(string $uri, ?string $baseUrl = null) {}
 
    public function getScheme(): ?string {}
 
    public function withScheme(?string $scheme): static {}
 
    public function getUser(): ?string {}
 
    public function withUser(?string $user): static {}
 
    public function getPassword(): ?string {}
 
    public function withPassword(?string $password): static {}
 
    public function getHost(): ?string {}
 
    public function withHost(?string $host): static {}
 
    public function getPort(): ?int {}
 
    public function withPort(?int $port): static {}
 
    public function getPath(): ?string {}
 
    public function withPath(?string $path): static {}
 
    public function getQuery(): ?string {}
 
    public function withQuery(?string $query): static {}
 
    public function getFragment(): ?string {}
 
    public function withFragment(?string $fragment): static {}
 
    public function equals(\Uri\Uri $uri, bool $excludeFragment = true): bool {}
 
    public function normalize(): static {}
 
    public function toNormalizedString(): string {}
 
    public function toString(): string {}
 
    public function resolve(string $uri): static {}
 
    public function __serialize(): array;
 
    public function __unserialize(array $data): void;
}
 
readonly class WhatWgUri implements \Uri\Uri
{
    private ?string $scheme;
    private ?string $user;
    private ?string $password;
    private ?string $host;
    private ?int $port;
    private ?string $path;
    private ?string $query;
    private ?string $fragment;
 
    /** @param array<int, \Uri\WhatWgError> $errors */
    public static function parse(string $uri, ?string $baseUrl = null, &$errors = null): ?static {}
 
    /** @param array<int, \Uri\WhatWgError> $softErrors */
    public function __construct(string $uri, ?string $baseUrl = null, &$softErrors = null) {}
 
    public function getScheme(): ?string {}
 
    public function withScheme(?string $scheme): static {}
 
    public function getUser(): ?string {}
 
    public function withUser(?string $user): static {}
 
    public function getPassword(): ?string {}
 
    public function withPassword(?string $password): static {}
 
    public function getHost(): ?string {}
 
    public function withHost(?string $host): static {}
 
    public function getPort(): ?int {}
 
    public function withPort(?int $port): static {}
 
    public function getPath(): ?string {}
 
    public function withPath(?string $path): static {}
 
    public function getQuery(): ?string {}
 
    public function withQuery(?string $query): static {}
 
    public function getFragment(): ?string {}
 
    public function withFragment(?string $fragment): static {}
 
    public function equals(\Uri\Uri $uri, bool $excludeFragment = true): bool {}
 
    public function normalize(): static {}
 
    public function toNormalizedString(): string {}
 
    public function toString(): string {}
 
    public function resolve(string $uri): static {}
 
    public function __serialize(): array {}
 
    public function __unserialize(array $data): void {}
}
 
enum WhatWgErrorType: int {
    case DomainToAscii = 0;
    case DomainToUnicode = 1;
    case DomainInvalidCodePoint = 2;
    case HostInvalidCodePoint = 3;
    case Ipv4EmptyPart = 4;
    case Ipv4TooManyParts = 5;
    case Ipv4NonNumericPart = 6;
    case Ipv4NonDecimalPart = 7;
    case Ipv4OutOfRangePart = 8;
    case Ipv6Unclosed = 9;
    case Ipv6InvalidCompression = 10;
    case Ipv6TooManyPieces = 11;
    case Ipv6MultipleCompression = 12;
    case Ipv6InvalidCodePoint = 13;
    case Ipv6TooFewPieces = 14;
    case Ipv4InIpv6TooManyPieces = 15;
    case Ipv4InIpv6InvalidCodePoint = 16;
    case Ipv4InIpv6OutOfRangePart = 17;
    case Ipv4InIpv6TooFewParts = 18;
    case InvalidUrlUnit = 19;
    case SpecialSchemeMissingFollowingSolidus = 20;
    case MissingSchemeNonRelativeUrl = 21;
    case InvalidReverseSoldius = 22;
    case InvalidCredentials = 23;
    case HostMissing = 24;
    case PortOfOfRange = 25;
    case PortInvalid = 26;
    case FileInvalidWindowsDriveLetter = 27;
    case FileInvalidWindowsDriveLetterHost = 28;
}
 
readonly class WhatWgError
{
    public string $context;
    public \Uri\WhatWgErrorType $type;
 
    public function __construct(string $context, \Uri\WhatWgErrorType $type) {}
}

First and foremost, the new URI parsing API contains a Uri\Uri interface which is implemented by two classes, Uri\Rfc3986Uri and Uri\WhatWgUri, representing RFC 3986 and WHATWG URIs, respectively. Having separate classes for the two standards makes it possible to indicate explicit intent at the type level that one specific standard is required. Actually, it may cause a security vulnerability to have wrong assumptions about the origin of a URI, as Daniel Stenberg (author of cURL) writes in one of his blog posts. That's why it's generally recommended to rely on one of the concrete URI implementations rather than the Uri\Uri interface itself for security-sensitive applications. In security-insensitive cases, Uri\Uri interface comes handy to be able to reference all URI implementations with the same type without having to list the accepted classes in a union type.

Both built-in URI implementations support instantiation via two methods:

• the constructor: It expects a required URI and an optional base URI parameter in order to support reference resolution. In case of an invalid URI, a Uri\InvalidUriException is thrown.
• a parse() factory method: It expects the same parameters as the constructor does, but in case of an invalid URI, null is returned instead of throwing an exception. Using this method is recommended for validating URIs and/or parsing URIs from untrusted input.

$uri = new Uri\Rfc3986Uri("https://example.com"); // An RFC 3986 URI instance is created
$uri = Uri\Rfc3986Uri::parse("https://example.com"); // An RFC 3986 URI instance is created
 
$uri = new Uri\Rfc3986Uri("invalid uri"); // A Uri/InvalidUriException is thrown
$uri = Uri\Rfc3986Uri::parse("invalid uri"); // null is returned in case of an invalid URI
 
$uri = new Uri\WhatWgUri("https://example.com"); // A WHATWG URL instance is created
$uri = Uri\WhatWgUri::parse("https://example.com"); // A WHATWG URL instance is created
 
$uri = new Uri\Rfc3986Uri("invalid uri"); // A Uri/InvalidUriException is thrown
$uri = Uri\Rfc3986Uri::parse("invalid uri", $errors); // null is returned, and an array of WhatWgError objects are passed by reference to $errors

The two built-in Uri\Uri implementations are readonly, and they have a respective private virtual property for each URI component. These URI components can be retrieved via getters, and immutable modification is possible via “wither” methods. While property hooks and/or asymmetric visibility would make it possible to get rid of the getters, the position of this RFC is to still go with regular get*() method calls as the conservative option, consistent with other internal interfaces. Hooked properties could be declared in interfaces, but since readonly properties are not supported, this possibility was rejected: the author of this RFC believes that it's more important to guarantee the immutability of URI implementations than to optimize performance via eliminating (getter) method calls.

$uri1 = new Uri\Rfc3986Uri("https://example.com");
$uri2 = $uri->withHost("test.com");
 
echo $uri1->getHost();                            // example.com
echo $uri2->getHost();                            // test.com

Besides accessors, Uri\Uri contains a toString() method too. This can be used for recomposing the URI components back to a string. Why such a method is necessary at all? It's because the recomposition process doesn't necessarily simply return the input URI, but it applies some modifications to it. The WHATWG standard specifically mandates the usage of quite some transformations (i.e. removal of extraneous / characters in the scheme, lowercasing some URI components, application of IDNA encoding). While some of the transformations are also required by default for RFC 3986, they are less frequent than for WHATWG.

$uri = new Uri\WhatWgUri("https://////example.com");
 
echo $uri->toString();                         // https://example.com

The attentive reader may have noticed that the examples used toString() instead of __toString(). This is a deliberate design decision not to add a __toString() method to the interface and its implementations, as doing so would cause incorrect results when using equality comparison (==). Given the following example:

$uri = new Uri\WhatWgUri("https://example.com");
 
var_dump($uri == 'HTTPS://example.com');

The output would be bool(false) if Uri\WhatWgUri contained a __toString() method, because of the $uri object being automatically converted to its string representation (https://example.com) which is then compared against HTTPS://example.com. However, as we will see in the next paragraphs, the two URIs should be indeed equal, as the result of normalization. Furthermore, equality of URIs usually disregards the fragment component, thus a https://example.com#foo URI would also yield a false positive result in the example.

As mentioned above, RFC 3986 has the notion of normalization, which is an optional process for canonizing different URIs identifying the same resource to the same URI. Therefore URI implementations may support normalization via the normalize() method. E.g. the https:///////EXAMPLE.com and the HTTPS://example.com/ URIs identify the same resource, so implementations may normalize both of them to https://example.com. If an implementation supports this process, it should apply some kind of normalization technique on the URI (i.e. case normalization, percent-encoding, normalization etc.) and return a new instance, otherwise the current, unmodified object can be returned. The toNormalizedString() method is a shorthand for $uri->normalize()->toString(), and it's useful when one needs the normalized string representation, but the URI components themselves don't have to be modified.

// Uri\Rfc3986Uri supports normalization
$uri = new Uri\Rfc3986Uri("https://EXAMPLE.COM/foo/../bar");
 
echo $uri->toString();                          // https://EXAMPLE.COM/foo/../bar"
echo $uri->normalize()->toString();             // https://example.com/bar
echo $uri->toNormalizedString();                // https://example.com/bar
 
// Uri\WhatWgUri normalizes the URI by default, therefore normalize() doesn't change anything
$uri = new Uri\WhatWgUri("https://EXAMPLE.COM/foo/../bar");
 
echo $uri->toString();                          // https://example.com/bar
echo $uri->normalize()->toString();             // https://example.com/bar
echo $uri->toNormalizedString();                // https://example.com/bar

Normalization is especially important when it comes to comparing URIs because it reduces the likelihood of false positive results, since URI comparison is based on checking whether the URIs represent the same resources. The Uri::equals() method can be used for comparing URIs. First, this method checks whether the called object and the URI instance received in the argument list has any parent-child relation, since it doesn't make sense to compare URIs of different standards. Then it normalizes and recomposes the URI represented by the object and the URI received in the argument list to a string, and checks whether the two strings match. By default, the fragment component is disregarded.

// A URI equals to another URI of the same standard that has the same string representation after normalization
new Uri\Rfc3986Uri("https://example.COM")->equals(new Uri\Rfc3986Uri("https://EXAMPLE.COM")));  // true
 
// A URI doesn't equal to another URI of a different standard even though they have the same string representation
new Uri\Rfc3986Uri("https://example.com/")->equals(new Uri\WhatWgUri("https://example.com/"));  // false

It should be noted that the equals() method only accepts an Uri\Uri instance, while it could also accept string URIs. It was a deliberate decision not to allow such arguments, because it would be unclear how the comparison works in this case: Should the passed in string be also normalized, or exact string match should be performed? Would the passed in URI string be parsed based on the same standard as the callee object? These are the questions which don't have to be answered when only the Uri\Uri parameter type is supported.

These pressing questions - combined with the fact that the construct is not supported in userland - led us not to overload the equality operator.

Last but not least, URIs support a resolve() method that is able to resolve potentially relative URIs with the current object as the base URI:

$uri = new Uri\Rfc3986Uri("https://example.com");
 
echo $uri->resolve("/foo")->toString();        //  https://example.com/foo

This is a shorthand for new get_class($uri)(“/foo”, $base->toString()).

Encoding and decoding special characters is a crucial aspect of URI parsing. For this purpose, both RFC 3986 and WHATWG use percent-encoding (i.e. the % character is encoded as %25). However, the two standards differ significantly in this regard:

RFC 3986 defines that “URIs that differ in the replacement of an unreserved character with its corresponding percent-encoded US-ASCII octet are equivalent”, which means that percent-encoded characters and their decoded form are equivalent. On the contrary, WHATWG defines URL equivalence by the equality of the serialized URLs, and never decodes percent-encoded characters, except in the host. This implies that percent-encoded characters are not equivalent to their decoded form (except in the host).

The difference between RFC 3986 and WHATWG comes from the fact that the point of view of a maintainer of the WHATWG specification is that webservers may legitimately choose to consider encoded and decoded paths distinct, and a standard cannot force them not to do so. This is a substantial BC break compared to RFC 3986, and it is actually a source of confusion among users of the WHATWG specification based on the large number of tickets related to this question.

For us, the biggest question is how a common interface can be designed in spite of this difference? Before trying to answer the question, it would be important to know some of the main use-cases of an URI abstraction:

• HTTP routers: They need the normalized and decoded representation of some HTTP components (i.e. path)
• HTTP clients: They use URIs as-is for sending requests, but they have some special use-cases (i.e. cookie handling) when a decoded URI component representation may be necessary for them
• SAML login: Service providers should do the least amount of processing of the login URL of identity providers in order not to break the SAML signature, therefore they work with the raw encoded URIs
• HTTP cache implementations: It's best to use a normalized URI in order not to store the same cache item for multiple URIs

As it can be seen based on just a few examples, an URI abstraction has to support many use-cases, out of which quite some are security-sensitive. That's why we should carefully evaluate how an URI interface should access the URI components. The position of this RFC is that the different use-cases can only be served with multiple accessor variants.

TBD

It would be very useful for an URI implementation to support direct query parameter manipulation. Actually, the WHATWG URL specification contains a URLSearchParams interface that could be used for the purpose. However, the position of this RFC is not to include this interface yet for the following reasons:

• Query string parsing is a fuzzy area, since there is no established rules how to parse query strings
• The URLSearchParams interface doesn't follow either RFC 1738, or RFC 3986
• The already large scope of the RFC would increase even more

For all these reasons, the topic of query parameter manipulation should be discussed as a followup to the current RFC.

PSR-7 UriInterface is currently the de-facto interface for representing URIs in userland. That's why it seems a good candidate for adoption at the first glance. However, the current RFC does not purse this mainly for the following reasons:

• PSR-7 strictly follows the RFC 3986 standard, and therefore only has a notion of "userinfo", rather than "user" and "password" which is used by the WHATWG specification.
• PSR-7's UriInterface have non-nullable method return types except for UriInterface::getPort() whereas WHATWG specifically allows null values.

Adding a WHATWG compliant URL parser to the standard library was originally attempted in 2023. The implementation used ADA URL parser as its parser backend which is known for its ultimate performance. At last, the proof of concept was abandoned due to some technical limitations that weren't possible to resolve.

Specifically, ADA is written in C++, and requires a compiler supporting C++17 at least. Despite the fact that it has a C wrapper, its tight compiler requirements would make it unprecedented, and practically impossible to add the URI extension to PHP as a required extension, because PHP has never had a C++ compiler dependency for the always enabled extensions, only optional extensions (like Intl) can be written in C++.

The firm position of this RFC is that an URL parser extension should always be available, therefore a different parser backend written in pure C should be found. Fortunately, Niels Dossche proposed PHP RFC: DOM HTML5 parsing and serialization not long after the experiment with ADA, and his work required bundling parts of the Lexbor browser engine. This library is written in C, and coincidentally contains a WHATWG compliant URL parsing submodule, therefore it makes it suitable to be used as the library of choice.

For parsing URIs according to RFC 3986, the URIParser library was chosen. It is a lightweight and fast C library with no dependencies. It uses the “new BSD license” which is compatible with the current PHP license as well as the PHP License Update RFC.

The capability provided by parse_url() is used for multiple purposes in the internal PHP source:

• SoapClient::_doRequest(): parsing the $location parameter as well as the value of the Location header
• FTP/FTPS stream wrapper: parse_url() is used for connecting to an URL, renaming a file, following the Location header
• FILTER_VALIDATE_URL: validating URLs
• SSL/TLS socket communication: parsing the target URL
• GET/POST session: accepting the session ID from the query string, manipulating the output URL to automatically include the session ID (Deprecate GET/POST sessions RFC

It would cause inconsistency and a security vulnerability if parsing of URIs based on the two specifications referred above were supported in userland, but the legacy parse_url() based behavior was kept internally without the possibility to use the new API. That's why the current RFC was designed with plugability in mind.

Specifically, supported parser backends would have to be registered by using a similar method how password hashing algorithms are registered. On one hand, this approach makes it possible for 3rd party extensions to leverage URI parser backends other than the built-in ones (i.e. support for ADA URL could also be added). But more importantly, an internal “interface” for parsing and handling URIs is defined this way so that it now becomes possible to configure the used backend for each use-case. Please note that URI parser backend registration is only supported for internal code: registering custom user-land implementations is not possible for now, mainly in order to prevent a possible new attack surface.

While it would sound natural to add a php.ini configuration option to configure the used parser backend globally, this option was rejected during the discussion period of the RFC because it would result in unsafe code that is controlled by global state: since any invoked piece of code can change the used parser backend, one should always check the current value of the config option before parsing URIs (and in case of libraries, the original option should also be reset after usage). Instead, the RFC proposes to add the following configuration options that only affect a single use-case:

• SoapClient::_doRequest(): a new optional $uriParserClass parameter is added accepting string or null arguments. Null represents the original (parse_url()) based method, while the new backends will be used when passing either Uri\Rfc3986Uri::class or Uri\WhatWgUri::class.
• FTP/FTPS stream wrapper: a new uri_parser_class stream context option is added
• FILTER_VALIDATE_URL: filter_* functions can be configured by passing a uri_parser_class key to the $options array
• SSL/TLS socket communication: a new uri_parser_class stream context option is added
• GET/POST session: since this feature is deprecated by (Deprecate GET/POST sessions RFC, no configuration is added.

There are certain file-handling functions that can already accept URIs as strings: these include file_get_contents(), file(), fopen(). As per the current proposal, the URI parser can be supplied in the $context parameter to these functions, but this approach is somewhat tedious, especially if the URI already had to be parsed previously (i.e. for validation purposes). Let's consider the following example:

$url = $_GET['url'];
validate_url($url);
 
$context = stream_context_create([
    "uri_parser_class" => \Uri\Rfc3986,
]);
$contents = file_get_contents($url, context: $context);

However, there are other much more convenient approaches, but the current RFC still goes with the current, less ergonomic one, as going either way would need more discussion, and a scope creep. The improvement possibilities include passing Uri\Uri instances to the functions in question, or converting URIs to streams based on Java's example.

The implementation of parse_url() is optimized for performance. This also means that it doesn't deal with validation properly and disregards some edge cases. A fully standard compliant parser will generally be slower than parse_url(), because it has to execute more code. Fortunately, this overhead is usually minimal thanks to the huge efforts of the maintainers of the Lexbor and the uriparser libraries.

According to the rough benchmarks, the following results were measured:

• parse_url(): 0.000208 sec
• Uri\Rfc3986Uri: 0.000311 sec
• Uri\WhatWgUri: 0.000387 sec

• parse_url(): 0.000962
• Uri\Rfc3986Uri: 0.000911 sec
• Uri\WhatWgUri: 0.000962 sec

Even though Go's standard library ships with a net/url package containing a url.Parse() function along with some utility functions, unfortunately it's not highlighted in the documentation which standard it conforms to. However, it's not very promising that the manual mentions the following sentence:

Trying to parse a hostname and path without a scheme is invalid but may not necessarily return an error, due to parsing ambiguities.

In Java, a URL class has been available from the beginning. Unfortunately, it's unclear whether it adheres to any URL standards. Speaking about its design, URL itself is immutable, and somewhat peculiarly, it contains some methods which can open a connection to the URL, or get its content.

Since Java 20, all of the URL constructors are deprecated in favor of using URI.toURL(). The URI class conforms to RFC 2396 standard.

NodeJS recently added support for a decent WHATWG URL compliant URL parser, built on top of the ADA URL parser project.

Python also comes with built-in support for parsing URLs, made available by the urllib.parse.urlparse and urllib.parse.urlsplit functions. According to the documentation, “these functions incorporate some aspects of both [the WHATWG URL and the RFC 3986 specifications], but cannot be claimed compliant with either”.

None.

The next minor PHP version (either PHP 8.5 or 9.0, whichever comes first).

SAPIs should adopt the new internal API for parsing URIs instead of using the existing php_url_parse*() API. Additionally, they should add support for configuring the URI parsing backend.

Extensions should adopt the new internal API for parsing URIs instead of using the existing php_url_parse*() API. Additionally, they should add support for configuring the URI parsing backend.

None.

• Support for new parser backends so that other libraries (like Ada URL, or cURL) could also be used in addition to uriparser and Lexbor.
• Support for an abstraction for manipulating query parameters, like URLSearchParams defined by WHATWG
• The parse_url() function can be deprecated at some distant point of time

Discussion thread: https://externals.io/message/123997

The vote requires 2/3 majority in order to be accepted.