PHP: rfc:switch-case-fallthrough

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====== PHP RFC: Explicit fallthrough for ''switch'' ======

  * Version: 0.1
  * Date: 2026-04-21
  * Author: Volker Dusch <volker@tideways-gmbh.com>, Tim Düsterhus <tim@tideways-gmbh.com>
  * Status: Draft
  * Target Version: PHP 8.6
  * Implementation: tbd

===== Introduction =====

This RFC proposes to add a ''fallthrough;'' marker to ''switch'' to prevent accidental fallthrough, helping to avoid bugs while making intentional fallthrough visible and explicit.

Empty case bodies, usually for a list of matches, will still require no ''fallthrough;''.

Implicit fallthrough is a source of bugs and potential confusion. Many languages found ways to clarify developer intent. When ''match'' was introduced fallthrough was omitted explicitly deliberately to avoid these issues, while php-src itself uses ''ZEND_FALLTHROUGH'' in its C code to allow for compiler warnings.

We propose to emit a ''E_DEPRECATED'' or ''E_WARNING'' if a non-empty ''case'' or ''default'' body falls through to the next case without an explicit ''fallthrough;''.

<PHP>
<?php

switch ($mode) {
    case 'create': // Non-empty case falls through to the next case, terminate the case with "fallthrough;" if this is intentional.
        audit('create');
    case 'update':
        persist($payload);
        break;
}

switch ($mode) {
    case 'create':
        audit('create');
        fallthrough; // Indicate the the fallthrough is intentional.
    case 'update':
        persist($payload);
        break;
}

switch ($extension) {
    case 'jpg': // No fallthrough needed here, intent is obvious
    case 'jpeg':
        return 'image/jpeg';
    case 'png':
        return 'image/png';
}

?>
</PHP>

===== Proposal =====

During compilation, each non-empty ''case'' / ''default'' body that is followed by another case in source order is checked.
If its last statement is not one of the following, the compiler emits an ''E_DEPRECATED'' or ''E_WARNING'':

  * <php>break;</php> / <php>break …;</php>
  * <php>continue;</php> / <php>continue …;</php>
  * <php>return;</php> / <php>return …;</php>
  * <php>throw …;</php>
  * <php>goto …;</php>
  * <php>fallthrough;</php>

The warning text used by the proof of concept is:

<code>
Non-empty case falls through to the next case, terminate the case with "fallthrough;" if this is intentional.
[ This behavior is deprecated in PHP 8.6 and will emit an E_WARNING in PHP 9.0. ]
</code>

Empty cases remain valid and continue to be used for grouped labels. They do not require ''fallthrough;''.

<PHP>
<?php

switch ($token) {
    case T_STRING:
        $value = normalize($value);
        fallthrough;
    case T_NAME_QUALIFIED:
        $result[] = validate($value);
        break;
}

switch ($extension) {
    case 'jpg':
    case 'jpeg':
        return 'image/jpeg';
    case 'png':
        return 'image/png';
}

?>
</PHP>

The proof of concept implements <php>fallthrough;</php> as a (useless) read of the <php>fallthrough</php> constant. This enables using this construct in older PHP versions when paired with a one-line polyfill:

<PHP>
<?php

if (!defined('fallthrough')) define('fallthrough', null);

?>
</PHP>

This keeps migration friction low for libraries, tooling, and mixed-version codebases.

The desired diagnostic level for that situation is left to a proposed secondary vote.


==== Examples ====

Empty ''case'' bodies fall through implicitly.

<PHP>
<?php

switch ($extension) {
    case 'jpg':
    case 'jpeg':
        return 'image/jpeg';
    case 'png':
        return 'image/png';
}

?>
</PHP>


PHP allows, but discourages, the <php>default:</php> to be in the non-final position in the switch. Fallthrough rules apply as if this would be a <php>case …:</php> statement.

<PHP>
<?php

switch ($extension) {
    case 'bin':
    default: // Uncommon but allowed, no fallthrough needed
    case 'binary':
        return 'application/octet-stream';
    case 'jpg':
    case 'jpeg':
        return 'image/jpeg';
}

switch ($extension) {
    default:
        log('unknown extension');
        fallthrough; // Fallthrough needed here
    case 'bin':
    case 'binary':
        return 'application/octet-stream';
    case 'jpg':
    case 'jpeg':
        return 'image/jpeg';
}

?>
</PHP>

==== Prior Art ====

  * C/C++: The ''-Wimplicit-fallthrough'' compiler warning and C23's [[https://en.cppreference.com/c/language/attributes/fallthrough|[[fallthrough]]]] attribute. This is the closest equivalent to what this RFC proposes.
  * Go: ''fallthrough'' is a reserved keyword; switch does not fall through by default.
  * C#: Requires explicit flow control (break, goto case, return, etc.) at the end of every non-empty case — implicit fallthrough is a compile error.
  * Swift: Has a fallthrough keyword; switch cases don't fall through by default.
  * Rust: match has no fallthrough concept at all.

===== Backward Incompatible Changes =====

Code that previously fell through silently from a non-empty case will now emit an <php>E_DEPRECATED</php> or <php>E_WARNING</php>. This is comparable to the existing warning for <php>continue;</php> targeting <php>switch()</php>: legal code remains legal, but likely mistakes become visible.

The proof of concept exposes <php>fallthrough</php> as a global constant. Whether the final implementation keeps that representation or uses a dedicated contextual/compiler-recognized form is an implementation detail; the source-level syntax proposed by this RFC is <php>fallthrough;</php>.

===== Proposed PHP Version(s) =====

PHP 8.6.

===== RFC Impact =====

==== To the Ecosystem ====

Tooling gets a canonical, machine-readable marker for intentional fallthrough instead of relying on comments like <php>/* fallthrough */</php> or project-specific conventions. Because <php>fallthrough;</php> is source-compatible with older runtimes via a trivial polyfill, analyzers and coding standards can adopt it immediately.

Existing usages and rules around fallthrough, excerpt:

  * [[https://cs.symfony.com/doc/rules/control_structure/no_break_comment.html]]
  * [[https://www.php-fig.org/per/coding-style/#52-switch-case-match]]
  * [[https://github.com/squizlabs/PHP_CodeSniffer/blob/master/src/Standards/PSR2/Sniffs/ControlStructures/SwitchDeclarationSniff.php#L196]]
  * [[https://github.com/nikic/PHP-Fuzzer/blob/23e440d79d353d5f42c9e41a50f7b4f930b85c00/src/Mutation/Mutator.php#L290]]
  * [[https://github.com/wikimedia/mediawiki-extensions-InlineComments/blob/dd5d2e291cb5c20920b9ba61965ed594ce7f3ea3/src/AnnotationTreeHandler.php#L128]]

==== To Existing Extensions ====

None.

==== To SAPIs ====

None.

===== Open Issues =====

None.

===== Future Scope =====

None.

===== Voting Choices =====

Primary Vote requiring a 2/3 majority to accept the RFC:

<doodle title="Emit a diagnostic on implicit fallthrough as outlined in the RFC?" voteType="single" closed="true" closeon="2026-01-01T15:30:00Z">
   * Yes
   * No
   * Abstain
</doodle>

Secondary vote requiring a simple majority, breaking ties towards E_WARNING:

<doodle title="Which diagnostic should an implicit switch falltrough emit?" voteType="single" closed="true" closeon="2026-01-01T15:30:00Z">
   * E_WARNING
   * E_DEPRECATED
   * Abstain
</doodle>

===== Patches and Tests =====

Proof of concept:

  * https://github.com/php/php-src/pull/21821

===== Implementation =====

If accepted, this section will be updated with the merge target version, implementing commit(s), and manual entry.

===== References =====

  * [[https://www.php.net/manual/en/control-structures.switch.php#:~:text=Note%3A%20Technically%20the%20default%20case%20may%20be%20listed%20in%20any%20order.%20It]]
  * [[https://wiki.php.net/rfc/match_expression_v2]]
  * [[https://wiki.php.net/rfc/continue_on_switch_deprecation]]

  * [[https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wimplicit-fallthrough_003d]]
  * [[https://clang.llvm.org/docs/AttributeReference.html#fallthrough]]
  * [[https://checkstyle.org/checks/coding/fallthrough.html#FallThrough]]
  * [[https://docs.swift.org/swift-book/documentation/the-swift-programming-language/statements#Fallthrough-Statement]]
  * [[https://go.dev/ref/spec#Fallthrough_statements]]


===== Rejected Features =====

==== Changing ''switch'' semantics ====

This RFC deliberately does not make implicit fallthrough an error and does not redesign ''switch''. The goal is to keep migration cost low.

===== Changelog =====

  * 2026-04-21: Initial draft.
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