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array_unique() with SORT_REGULAR returns duplicate values #20262
Description
Description
The following code:
<?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
)
Demonstrations:
- simple array of strings: https://3v4l.org/M5lcP
- array of objects: https://3v4l.org/5kr1t
- array of arrays: https://3v4l.org/0HVoV
Root Cause
The algorithm:
- Sort array using comparison function from
php_get_data_compare_func_unstable() - Walk through sorted array comparing only adjacent elements
- Delete duplicates when adjacent elements compare equal
The bug:
SORT_REGULAR uses zend_compare() which calls zendi_smart_strcmp() for string comparisons. This function has non-transitive behavior when mixing numeric and non-numeric strings:
"5" < "10"→ true (numeric comparison: 5 < 10)"10" < "3A"→ true (lexicographic: "1" < "3")"3A" < "5"→ true (lexicographic: "3" < "5") Creates a cycle!
Because the comparison is non-transitive, sorting algorithms (which require transitive comparisons) produce inconsistent results depending on input order.
The deduplication walks through comparing adjacent elements:
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" → deleteThe root issue: Non-transitive comparisons break the sorting algorithm's guarantee that equal values will be grouped together. The adjacent-only comparison is correct - but it requires the array to be properly sorted first, which requires transitive comparisons.
Comparison with SORT_STRING
<?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 ✓ CorrectSORT_STRING uses lexical comparison without numeric extraction, so duplicates stay grouped.
Workaround
For simple arrays of scalar values, you can use array_unique with default SORT_STRING flag.
<?php
$unique = array_unique($array, SORT_STRING);For arrays or objects.
$uniqueAddr = [];
foreach ($addresses as $addr) {
if (! in_array($addr, $uniqueAddr)) {
$uniqueAddr[] = $addr;
}
}PHP Version
PHP 8.4.13 (cli) (built: Sep 26 2025 00:45:36) (NTS clang 15.0.0)
Copyright (c) The PHP Group
Built by Laravel Herd
Zend Engine v4.4.13, Copyright (c) Zend Technologies
with Zend OPcache v8.4.13, Copyright (c), by Zend Technologies
Activity
devnexen commentedon Oct 23, 2025
It is borderline feature request though, it behaves as expected that would be a BC change IMHO. cc @nielsdos just in case.
jmarble commentedon Oct 23, 2025
I think you mean borderline user error ;-)
I really can't imagine a world where anyone would call
array_unique($units, SORT_REGULAR)and expect to get duplicates in the returned array. It's definitely a bug unfortunately. And I fear many users out there are just blissfully unaware.jmarble commentedon Oct 23, 2025
Actually, @devnexen perhaps now is the time we consider killing two birds with one stone? Provide a proper type-strict implementation of
array_unique(), without resorting to an O(n²) style implementation ofin_array(). My thought is if theSORT_REGULARflag can properly find uniqueness within an array of mixed types, a flag likeSORT_STRICTshould be possible -- but I digress.devnexen commentedon Oct 23, 2025
Precisely why I am calling in someone who knows a lot better array internals :) I have the feeling it might be performance costly to fix it with the actual implementation or would need a good rewrite. But I might be wrong.
macropay-solutions commentedon Oct 23, 2025
@devnexen @jmarble the issue is the sort of mixed types not the way it compares the sorted result.
Basically sort acts as a groupBy same values which then is walked through to eliminate the duplicates.
devnexen commentedon Oct 23, 2025
either way I kept the bug label, someone will answer to your request shortly.
jmarble commentedon Oct 23, 2025
@macropay-solutions you're correct that sorting is the primary issue, but the adjacent-only comparison is what allows the bug to manifest.
Add uniqueStrings() method to collections
Add uniqueStrings() method to collections
Add uniqueStrings() method to collections
45 remaining items
marius-ciclistu commentedon Dec 28, 2025
Is this issue going to be buried?
jmarble commentedon Dec 28, 2025
I've published two PRs that fix the issue with SORT_REGULAR.
I also have an RFC for a new SORT_STRICT flag that provides a way to work around the issue. I will be moving the RFC to discussion maybe next week.
shaedrich commentedon Dec 28, 2025
@marius-ciclistu At least there are some PRs:
php/php-srcphp/doc-enThe source code fixes already got some reviews from PHP team members
marius-ciclistu commentedon Dec 28, 2025
@shaedrich I know. But they are not merged.
shaedrich commentedon Dec 28, 2025
They've been worked on last in November. Keep your hopes up
Add note about SORT_REGULAR + numeric strings bug to array_unique() (#…