<?php
$time = microtime();
$a = range(1,100000);
foreach ($a as $i) {
isset($a['asd']);
}
echo microtime() - $time;
echo "\n";
$time = microtime();
$a = range(1,100000);
foreach ($a as $i) {
array_key_exists('asd', $a);
}
echo microtime() - $time;