[php-src] Issue #20262: array_unique() with SORT_REGULAR returns duplicate values

Issue: https://github.com/php/php-src/issues/20262 Author: jmarble ### Description The following code: ```php <?php $units = ['5', '10', '5', '3A', '5', '5']; $unique = array_unique($units, SORT_REGULAR); print_r($unique); ``` Resulted in this output: ``` Array ( [0] => 5 [1] => 10 [3] => 3A [4] => 5 ) ``` But I expected this output instead: ``` Array ( [0] => 5 [1] => 10 [3] => 3A ) ``` **The value "5" appears at both indices [0] and [4] in the result.** This array represents apartment/unit numbers (e.g., Unit 5, Unit 10, Unit 3A), which is a standard format in building management systems where letter suffixes denote sub-units or variations. Demonstration: https://3v4l.org/M5lcP --- ## Root Cause From analyzing PHP source code (ext/standard/array.c, Zend/zend_operators.c): **The algorithm:** 1. Sort array using zendi_smart_strcmp() which calls is_numeric_string_ex() 2. Walk through sorted array comparing **only adjacent elements** 3. Delete duplicates from original array **The bug:** is_numeric_string_ex() extracts leading numeric portions: - "3A" → extracts 3 - "5" → extracts 5 - "10" → extracts 10 - Compares numerically: 3 < 5 < 10 However, unstable sort produces: ``` Sorted: ["5", "10", "10", "3A", "5", "5"] ``` The "3A" (numeric value 3) ends up AFTER "10" instead of before "5", separating the duplicate "5" values. The deduplication walks through comparing adjacent elements: ```c lastkept = position_0; // "5" position_1 "10" != "5" → keep, lastkept = position_1 position_2 "10" == "10" → delete position_3 "3A" != "10" → keep, lastkept = position_3 position_4 "5" != "3A" → keep ← Bug! Never compared to position_0 position_5 "5" == "5" → delete ``` **The flaw:** The algorithm only compares with lastkept (last unique value), not with all previous values. Position 4's "5" is never compared back to position 0's "5". **Source files:** - ext/standard/array.c - PHP_FUNCTION(array_unique) - Zend/zend_operators.c - zendi_smart_strcmp(), is_numeric_string_ex() --- ## Comparison with SORT_STRING ```php <?php $units = ['5', '10', '5', '3A', '5', '5']; echo count(array_unique($units, SORT_REGULAR)) . "\n"; // 4 ✗ Wrong echo count(array_unique($units, SORT_STRING)) . "\n"; // 3 ✓ Correct ``` SORT_STRING uses lexical comparison without numeric extraction, so duplicates stay grouped. --- ## Workaround ```php <?php $unique = array_unique($array, SORT_STRING); ``` ### PHP Version ```plain PHP 8.4.13 (cli) (built: Sep 26 2025 00:45:36) (NTS clang 15.0.0) Copyright (c) The PHP Group Built by Laravel Herd Zend Engine v4.4.13, Copyright (c) Zend Technologies with Zend OPcache v8.4.13, Copyright (c), by Zend Technologies ``` ### Operating System _No response_