<?php


function trimAll(&$array) {
    foreach($array as $k => $v) {
        if(isset($array[$k]) && is_string($array[$k]))
            $array[$k]=&trim($array[$k]);
    }
}


$a='  aaa ';
$b='  bbb ';
$c='  ccc ';
trimAll(array($a,$b,$c));


echo $a;



?>

Fatal error: Only variables can be passed by reference on line 15