gistfile1.txt

As the manual says, "If two members compare as equal, their order in the sorted array is undefined." This means that the sort used is not "stable" and may change the order of elements that compare equal. 

Sometimes you really do need a stable sort. For example, if you sort a list by one field, then sort it again by another field, but don't want to lose the ordering from the previous field. In that case it is better to use usort with a comparison function that takes both fields into account, but if you can't do that then use the function below. It is a merge sort, which is guaranteed O(n*log(n)) complexity, which means it stays reasonably fast even when you use larger lists (unlike bubblesort and insertion sort, which are O(n^2)). 

<?php 
function mergesort(&$array, $cmp_function = 'strcmp') { 
    // Arrays of size < 2 require no action. 
    if (count($array) < 2) return; 
    // Split the array in half 
    $halfway = count($array) / 2; 
    $array1 = array_slice($array, 0, $halfway); 
    $array2 = array_slice($array, $halfway); 
    // Recurse to sort the two halves 
    mergesort($array1, $cmp_function); 
    mergesort($array2, $cmp_function); 
    // If all of $array1 is <= all of $array2, just append them. 
    if (call_user_func($cmp_function, end($array1), $array2[0]) < 1) { 
        $array = array_merge($array1, $array2); 
        return; 
    } 
    // Merge the two sorted arrays into a single sorted array 
    $array = array(); 
    $ptr1 = $ptr2 = 0; 
    while ($ptr1 < count($array1) && $ptr2 < count($array2)) { 
        if (call_user_func($cmp_function, $array1[$ptr1], $array2[$ptr2]) < 1) { 
            $array[] = $array1[$ptr1++]; 
        } 
        else { 
            $array[] = $array2[$ptr2++]; 
        } 
    } 
    // Merge the remainder 
    while ($ptr1 < count($array1)) $array[] = $array1[$ptr1++]; 
    while ($ptr2 < count($array2)) $array[] = $array2[$ptr2++]; 
    return; 
} 
?>
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