In php how does usort() function works

Question

I have looked at the php documentation, tutorials online and none of them how usort is actually working. I have an example i was playing with below.

$data = array(      array('msg' => 'some text','month' => 11,'level' => 10),      array('msg' => 'some text','month' => 5,'level' => 10),      array('msg' => 'some text','month' => 8,'level' => 10),      array('msg' => 'some text','month' => 12,'level' => 10),      array('msg' => 'some text','month' => 2,'level' => 10),      array('msg' => 'some text','month' => 3,'level' => 10),      array('msg' => 'some text','month' => 4,'level' => 10),      array('msg' => 'some text','month' => 7,'level' => 10),      array('msg' => 'some text','month' => 10,'level' => 10),      array('msg' => 'some text','month' => 1,'level' => 10),      array('msg' => 'some text','month' => 6,'level' => 10),      array('msg' => 'some text','month' => 9,'level' => 10)  ); 

I wanted to be able to sort the months from 12 to 1 (since their unorganized) through some help this was the solution

function cmp($a, $b) {     if ($a["month"] == $b["month"])      {        return 0;     }     return ($a["month"] < $b["month"]) ? -1 : 1; }  usort($data, "cmp"); 

but i dont understand how the function cmp sorts the array. i tried printing out each variable $a and $b like this:

function cmp($a, $b) {    echo "a: ".$a['month']."<br/>";    echo " b: ".$b['month']."<br/>";    echo "<br/><br/>"; } 

and the output was

a: 3 b: 5  a: 9 b: 3  a: 3 b: 8  a: 6 b: 3  a: 3 b: 12  a: 1 b: 3  a: 3 b: 2  a: 10 b: 3  a: 3 b: 11  a: 7 b: 3  a: 4 b: 3  a: 12 b: 2  a: 5 b: 12  a: 12 b: 11  a: 8 b: 12  a: 5 b: 8  a: 2 b: 11  a: 6 b: 9  a: 7 b: 6  a: 6 b: 4  a: 10 b: 6  a: 1 b: 6  a: 9 b: 4  a: 7 b: 1  a: 10 b: 7 

it makes no sense to how the sort is working and why cmp($a, $b) is used. i have tried to print out all its processes as you can see but have not come to any solution to how it all works..

thanks

Answer 1

The callback provided to the sorting functions in PHP have three return values:

0:  both elements are the same -1 (<0): the first element is smaller than the second 1 (>0):  the first element is greater 

Now, usort probably uses some kind of quicksort or mergesort internally. For each comparison it calls your callback with two elements and then decides if it needs to swap them or not.

Answer 2

The function cmp itself doesn't do the sorting. It just tells usort if a value is smaller, equal or greater than another value. E.g. if $a = 5 and $b = 9 it will return 1 to indicate that the value in $b is greater than the one in $a.

Sorting is done by usort.