Sahn's example almost works but has a small error. Try it like this if you need to prepend something to the array without the keys being reindexed and/or need to prepend a key value pair, you can use this short function: 
<?php 
function array_unshift_assoc(&$arr, $key, $val) 
{ 
 $arr = array_reverse($arr, true); 
 $arr[$key] = $val; 
 return = array_reverse($arr, true); 
} 
?>