<?
function cmp($a, $b)
{

$arrComp=array('c'=>0,'s'=>1,'a'=>2,'b'=>3,'k'=>4) ;

if (($arrComp[$a]) == ($arrComp[$b])) {
return 0;
}
return (($arrComp[$a]) < ($arrComp[$b])) ? -1 : 1;
}


$a = array('b','s','a','c','k','b','s','a','c','k');
usort($a, "cmp");

foreach ($a as $key => $value) {
echo "
$key -> $value\n";
}

?>
ti stampa
0 -> c
1 -> c
2 -> s
3 -> s
4 -> a
5 -> a
6 -> b
7 -> b
8 -> k
9 -> k