Case-insensitive substr_compare, ksort, krsort are still locale-dependent #24552

Description

The following code:

<?php
foreach (['C', 'tr_TR.UTF-8'] as $locale) {
    if (setlocale(LC_CTYPE, $locale) === false) {
        throw new RuntimeException("Locale unavailable: $locale");
    }

    $values = ['I', 'h'];
    sort($values, SORT_STRING | SORT_FLAG_CASE);
    $keys = ['I' => 0, 'h' => 0];
    ksort($keys, SORT_STRING | SORT_FLAG_CASE);

    echo $locale, "\n";
    echo '  stripos:        ', var_export(stripos('I', 'i'), true), "\n";
    echo '  strncasecmp:    ', strncasecmp('I', 'i', 1), "\n";
    echo '  substr_compare: ', substr_compare('I', 'i', 0, 1, true), "\n";
    echo '  sort:           ', implode(',', $values), "\n";
    echo '  ksort:          ', implode(',', array_keys($keys)), "\n";
}

Resulted in this output:

C
  stripos:        0
  strncasecmp:    0
  substr_compare: 0
  sort:           h,I
  ksort:          h,I
tr_TR.UTF-8
  stripos:        0
  strncasecmp:    0
  substr_compare: -32
  sort:           h,I
  ksort:          I,h

But I expected this output instead:

C
  stripos:        0
  strncasecmp:    0
  substr_compare: 0
  sort:           h,I
  ksort:          h,I
tr_TR.UTF-8
  stripos:        0
  strncasecmp:    0
  substr_compare: 0
  sort:           h,I
  ksort:          h,I

Reproduction link

PHP 8.2 switched strtolower(), stripos(), sort() with SORT_STRING | SORT_FLAG_CASE # and related functions to ASCII case conversion, but substr_compare() with $case_insensitive = true and ksort()/krsort() with SORT_STRING | SORT_FLAG_CASE still follow LC_CTYPE.

The output above is from PHP 8.2.35, 8.3.36, 8.4.27 and 8.5.12. PHP 8.1.34 prints the same, except that stripos() and sort() also follow the locale (false and I,h). 8.2 changed those two but not substr_compare() or ksort().

for v in 8.1 8.2 8.3 8.4 8.5; do
  docker run --rm -i "php:$v-cli" sh -c '
    apt-get update -qq >/dev/null &&
    apt-get install -y -qq locales >/dev/null 2>&1 &&
    localedef -i tr_TR -f UTF-8 tr_TR.UTF-8 &&
    php -v | head -n 1 &&
    php' < repro.php
done

The 8.2 UPGRADING notes say sorting with SORT_FLAG_CASE uses ASCII case conversion, and the strtolower-ascii RFC says case-insensitive substr_compare() already did.

On master, substr_compare() calls zend_binary_strncasecmp_l(), and php_array_key_compare_string_case_unstable_i(), which ksort() and krsort() use, calls zend_binary_strcasecmp_l().

Switching these two call sites to zend_binary_strncasecmp() and zend_binary_strcasecmp() would give the documented behavior.

See #7506.

PHP Version

PHP 8.5.12 (cli) (built: Oct 21 2026 15:53:40) (NTS)
Copyright (c) The PHP Group
Built by https://github.com/docker-library/php
Zend Engine v4.5.12, Copyright (c) Zend Technologies
    with Zend OPcache v8.5.12, Copyright (c), by Zend Technologies

Operating System

Debian GNU/Linux 13 (trixie), glibc 2.41 (php:8.5-cli Docker image)