[felixbuenemann]

setlocale() changes the locale of the whole process, so in FrankenPHP:

• a request uses the locale set by requests running at the same time;
• the end of a request resets the locale of all other requests to C;
• concurrent calls can crash the process, because setlocale() isn't thread-safe.

This PR replaces setlocale() with a version that gives each thread its own locale, using the POSIX newlocale() and uselocale() functions. It's the same approach as the getenv()/putenv() sandbox. PHP code doesn't need to change.

Related: #1941 (gettext mixing languages, where this approach was first discussed), #1737 (setlocale() named among the MT-unsafe functions WordPress calls). This also updates the worker-mode note added in #2779.

Reproduction

The new tests fail on main. With the PHP 8.5 ZTS build on macOS, all three fail (localeconv()['decimal_point'] returning another request's ,, C instead of de_DE.UTF-8, etc.), and the test binary aborted (SIGABRT) in 2 of 3 runs.

In a real application (TYPO3 patched for worker-mode, alternating German and English pages, 8 concurrent connections), main rendered 171 of 4000 pages with the wrong locale, then aborted. With this PR, all 4000 pages were correct.

How it works

• setlocale() is replaced in MINIT through CG(function_table), like getenv() and putenv(). Everything is in the new threadlocale.c, and the changes to frankenphp.c are small.
• Each combination of category locales is created once with newlocale(), under a mutex, and shared by all threads. Threads only switch between these objects with uselocale(), and the locale-dependent C functions (printf(), localeconv(), strftime(), strcoll()…) follow it.
• The locale objects are never freed, for three reasons:
  • other threads may still use them;
  • a process only needs a few, since there is one per distinct combination;
  • on macOS, freelocale() crashes while another thread calls newlocale().
• Queries (setlocale(LC_ALL, '0')) return the same names as the C library, including composite LC_ALL names in each platform's own format, which differs between glibc, musl and macOS/BSD. So setlocale(LC_ALL, $saved) still restores all categories.
• The argument handling is copied from ext/standard/string.c, gated by PHP_VERSION_ID so it matches each version:
  • 8.2–8.4 convert the arguments one at a time;
  • 8.5 type-checks all arguments first;
  • 8.6+ also rejects NUL bytes, and accepts an array only as the sole locale argument.
• Classic mode: at RSHUTDOWN, the thread switches back to the process locale. PHP's own reset of the process locale to C no longer runs.
• Worker mode: each request starts with the locale the worker script set before calling frankenphp_handle_request().
• Windows: PHP already keeps the locale per thread there, so setlocale() isn't replaced. In worker mode, the locale the worker script set is restored after each request, as on other platforms.

Limitation: C code that calls the C library's setlocale() directly, in a PHP extension for instance, still changes the locale of the whole process.

Tests

• TestSetlocale, TestSetlocale_worker and TestSetlocaleBeforeHandlingRequests_worker run concurrent requests that switch between locales. They pass with -race.
• A script with 30 locale edge cases and 31 argument edge cases produced output identical to the native PHP CLI. The edge cases include invalid names, '0', '', null, arrays, LC_ALL composites, wrong types, strict_types, and NUL bytes. Platforms:
  • PHP 8.3, 8.4, 8.5, 8.6.0RC5 and master on macOS;
  • PHP 8.2 and master on Debian (glibc 2.41);
  • PHP 8.5 on Debian and Alpine (musl).
• The full suite (go test -race ./..., library and Caddy module) passes with PHP 8.5 on macOS. The only exception is TestExecuteScriptCLISignals, which crashes the same way on main there.
• Not tested:
  • gettext, the case in docs: Document gettext incompatibility #1941: the build used here has no gettext extension. glibc's gettext follows uselocale();
  • Windows: only by CI, which runs the same tests there.

Performance

• On macOS, changing the locale took 108 µs with the native setlocale(), which reloads the locale data on every call. With this PR it takes 0.15 µs, because the locale objects are reused.
• TYPO3 pages that alternate between languages got about 16% higher requests/s.

Relation to php-src

The same change was proposed for PHP itself in php/php-src#20531. Reviewers there found the uselocale() approach reasonable, but the PR was closed in August 2026 in favour of a broader ZTS rework. Until PHP has thread-local locales, this fixes the problem for FrankenPHP users on all supported PHP versions. Once PHP has them, the override can be gated by PHP_VERSION_ID and removed later.

[dunglas]

Shouldn't this be changed in PHP directly?

[felixbuenemann]

Shouldn't this be changed in PHP directly?

Ideally it would, but work started on it before was abandoned and it could land earliest in php 8.7 which is a year a way. What would make sense is trying to get the php-specific bits into 8.7 in a parallel PR. However this code provides a solution for 8.2+ when used with frankenphp.