note 108116 added to function.usort

When trying to do some custom sorting with objects and an anonymous function it wasn't entirely clear how this usort function works. I think it probably uses a quicksort in the background. Basically it actually moves the $b variable up or down in respect to the $a variable. It does NOT move the $a variable inside the callback function. This is key to getting your logic right in the comparisons. If you return -1 that moves the $b variable down the array, return 1 moves $b up the array and return 0 keeps $b in the same place. To test I cut down my code to sorting a simple array from highest priority to lowest. <?php $priorities = array(5, 8, 3, 7, 3); usort($priorities, function($a, $b) { if ($a == $b) { echo "a ($a) is same priority as b ($b), keeping the same\n"; return 0; } else if ($a > $b) { echo "a ($a) is higher priority than b ($b), moving b down array\n"; return -1; } else { echo "b ($b) is higher priority than a ($a), moving b up array\n"; return 1; } }); echo "Sorted priorities:\n"; var_dump($priorities); ?> Output: b (8) is higher priority than a (3), moving b up array b (5) is higher priority than a (3), moving b up array b (7) is higher priority than a (3), moving b up array a (3) is same priority as b (3), keeping the same a (8) is higher priority than b (3), moving b down array b (8) is higher priority than a (7), moving b up array b (8) is higher priority than a (5), moving b up array b (8) is higher priority than a (3), moving b up array a (5) is higher priority than b (3), moving b down array a (7) is higher priority than b (5), moving b down array Sorted priorities: array(5) { [0]=> int(8) [1]=> int(7) [2]=> int(5) [3]=> int(3) [4]=> int(3) } ---- Server IP: 69.147.83.197 Probable Submitter: 123.255.40.13 ---- Manual Page -- http://www.php.net/manual/en/function.usort.php Edit -- https://master.php.net/note/edit/108116 Del: integrated -- https://master.php.net/note/delete/108116/integrated Del: useless -- https://master.php.net/note/delete/108116/useless Del: bad code -- https://master.php.net/note/delete/108116/bad+code Del: spam -- https://master.php.net/note/delete/108116/spam Del: non-english -- https://master.php.net/note/delete/108116/non-english Del: in docs -- https://master.php.net/note/delete/108116/in+docs Del: other reasons-- https://master.php.net/note/delete/108116 Reject -- https://master.php.net/note/reject/108116 Search -- https://master.php.net/manage/user-notes.php