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

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
 
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';
}
 
?>

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:

• break; / break …;
• continue; / continue …;
• return; / return …;
• throw …;
• goto …;
• fallthrough;

The warning text used by the proof of concept is:

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. ]

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

<?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';
}
 
?>

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

<?php
 
if (!defined('fallthrough')) define('fallthrough', null);
 
?>

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.

Empty case bodies fall through implicitly.

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

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

<?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';
}
 
?>

• C/C++: The -Wimplicit-fallthrough compiler warning and C23's [[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.

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

The proof of concept exposes fallthrough 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 fallthrough;.

PHP 8.6.

Tooling gets a canonical, machine-readable marker for intentional fallthrough instead of relying on comments like /* fallthrough */ or project-specific conventions. Because fallthrough; 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

None.

None.

None.

None.

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

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

Proof of concept:

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

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

• 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

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

• 2026-04-21: Initial draft.