Should array_udiff be comparing elements from the same array?

Hi. Was testing array_udiff against some complex behaviour. Had an oddity. Reduced the code to a simple test ... 3v4l.org/XON0v <?php $array1 = array(1, 2, 3); $array2 = array(3, 4, 5); print_r( array_udiff( $array1, $array2, function ($a, $b) { static $tests = 0; echo 'array_udiff ', ++$tests, ' ', $a, ' vs ', $b, PHP_EOL; return ($a < $b) ? -1 : (($a > $b) ? 1 : 0); } ) ); The output is : array_udiff 1 1 vs 2 array_udiff 2 2 vs 3 array_udiff 3 3 vs 4 array_udiff 4 4 vs 5 array_udiff 5 1 vs 3 array_udiff 6 1 vs 2 array_udiff 7 2 vs 3 array_udiff 8 2 vs 3 array_udiff 9 3 vs 3 Array ( [0] => 1 [1] => 2 ) Tests 1 and 4 suggests that elements from with a single array are being compared. Is this expected behaviour? This seems wrong when you use a nested array and each set uses a different key name. For example. <?php $array1 = array(array('A' => 1), array('A' => 2), array('A' => 3)); $array2 = array(array('B' => 3), array('B' => 4), array('B' => 5)); print_r( array_udiff( $array1, $array2, function ($a, $b) { static $tests = 0; echo 'array_udiff ', ++$tests, ' ', $a['A'], ' vs ', $b['B'], PHP_EOL; return ($a['A'] < $b['B']) ? -1 : (($a['A'] > $b['B']) ? 1 : 0); } ) ); With this, you now get errors. array_udiff 1 array ( 'A' => 1, ) vs array ( 'A' => 2, ) Notice: Undefined index: B in /Users/richardquadling/Library/Preferences/PhpStorm2017.3/scratches/scratch_5.php on line 14 There is a user note http://php.net/manual/en/function.array-udiff.php#86220 "It is not stated, by this function also diffs array1 to itself, removing any duplicate values..." This would seem to be the cause bit is clearly not the intent of the function. -- Richard Quadling