<?php
$a = array('aa'=>'a','bb'=>'b','cc'=>'c');
$t1=microtime(true);
array_shift($a);
echo 'array_shift($a):'.((microtime(true)-$t1)*1000).'<br><br>';
$t2=microtime(true);
unset($a['aa']);
echo "unset(\$a['aa']):".((microtime(true)-$t2)*1000).'<br>';
?>