Description:
------------
on an array with collection of the same object
the object have 2 property a et b
Exemple:
[
  object:{a=3, b=2}
  object:{a=0, b=2}
  object:{a=2, b=3}
  object:{a=1, b=2}


If you make a first sort with the good fonction and usort on property a, you obtain 

[
  object:{a=0, b=2}
  object:{a=1, b=2}
  object:{a=2, b=3}
  object:{a=3, b=2}
]
The result is that you intend

Now if you make a second sort with another function and usort on property b, you obtain
[
  object:{a=3, b=2}
  object:{a=1, b=2}
  object:{a=0, b=2}
  object:{a=2, b=3}
]
The result is not you can wait. I think that's because the read of the array is make in inverse way.

A possible solution is to mark the placed elements. And not move elements not tested before or after them the marked element.

Best Regards



Test script:
---------------
<?php
function compa($a, $b){ if($a['a']>$b['a']) return 1; else return ($a['a']<$b['a'])?-1:0; }
function compb($a, $b){ if($a['b']>$b['b']) return 1; else return ($a['b']<$b['b'])?-1:0; }
$a = array(array('a'=>1, 'b'=>2), array('a'=>2, 'b'=>3), array('a'=>3, 'b'=>2), array('a'=>0, 'b'=>2), array('a'=>4, 'b'=>2));
usort($a, 'compa');
print_r($a);
usort($a, 'compb');
print_r($a);
?>


Expected result:
----------------
Array
(
    [0] => Array
        (
            [a] => 0
            [b] => 2
        )
    [1] => Array
        (
            [a] => 1
            [b] => 2
        )
    [2] => Array
        (
            [a] => 3
            [b] => 2
        )
    [3] => Array
        (
            [a] => 2
            [b] => 3
        )
)


Actual result:
--------------
Array
(
    [0] => Array
        (
            [a] => 3
            [b] => 2
        )

    [1] => Array
        (
            [a] => 1
            [b] => 2
        )

    [2] => Array
        (
            [a] => 0
            [b] => 2
        )

    [3] => Array
        (
            [a] => 2
            [b] => 3
        )

)