proc_close() returns raw signal number, indistinguishable from normal exit code

Description

On non-Windows systems, when a child process opened via proc_open() is killed by a signal, proc_close() returns the raw signal number (e.g. 11 for SIGSEGV) rather than following the Unix convention of 128 + signal (e.g. 139).

This makes it impossible for PHP userland code to distinguish between "process exited normally with code 11" and "process was killed by SIGSEGV".

Underlying cause

Internally, proc_close() calls waitpid() which returns an encoded status. The C macros WIFEXITED()/WEXITSTATUS() and WIFSIGNALED()/WTERMSIG() exist to decode this, but proc_close() does not use them to differentiate the two cases. It effectively passes back a value that loses the signal/exit distinction.

PHP's own pcntl_waitpid() + pcntl_wifexited() + pcntl_wtermsig() handle this correctly, but pcntl is not always available, and proc_close() is the standard way to get the exit status of a proc_open() process.

Reproduction

<?php

// Child script that triggers SIGSEGV
$child = '<?php posix_kill(posix_getpid(), 11);';

$process = proc_open(
    [PHP_BINARY, '-r', $child],
    [],
    $pipes
);

$exitCode = proc_close($process);

// Expected: 139 (128 + 11, following bash/Unix convention)
// Actual: 11 (raw signal number, same as a normal exit code)
echo "Exit code: $exitCode\n";
echo "Is this SIGSEGV or a normal exit(11)? Impossible to tell.\n";

Expected behavior

proc_close() should return 128 + signal_number when the child was killed by a signal, consistent with how bash and other Unix shells report signal termination via $?. This would allow userland code to detect crashes.

Alternatively, expose the signal information through a separate mechanism (e.g. an optional by-reference parameter, or a new proc_exit_status() function).

Real-world impact

This affects tools that use proc_open() to spawn child processes, notably composer/xdebug-handler which restarts PHP processes. When the child crashes (e.g. due to a PHP JIT bug triggering SIGSEGV), the parent receives exit code 11 with no way to detect it was a signal — leading to silent, misleading failures in tools like Psalm, PHPStan, and Composer.

• Related upstream fix: https://github.com/composer/xdebug-handler/pull/162
• Downstream report: https://github.com/vimeo/psalm/issues/11679

PHP Version

Tested on PHP 8.4 and 8.5, but the behavior has existed since proc_open was introduced.

Operating System

Linux / macOS (any non-Windows POSIX system)

Trace proc_close() on non-Windows systems through waitpid() and the WIFEXITED(), WEXITSTATUS(), WIFSIGNALED(), and WTERMSIG() cases described in the issue. Run the provided proc_open() reproduction with a SIGSEGV child and verify that signal termination is distinguishable from a normal exit code while preserving expected behavior on other platforms.