<?
    
    $keys = array('--m2' => 1, -500 => 1, 0 => 1, 1000 => 1, '--m1' => 1, 5000 => 1, );
    
    ksort($keys, SORT_STRING);
    
    foreach($keys as $k => $v){
        echo $k . "\n";
    }
    
    ?>

    --m1
--m2
-500
0
1000
5000