isset vs array_key_exists

<?php

function prepare($num) {
    echo "preparing array with $num elements ... ";
    $array = array();
    for ($i=0; $i<$num; $i++) {
        $array[uniqid('', true)] = true;
    }
    echo 'OK', PHP_EOL;
    return $array;
}

function test($num, $cycles, $tries) {
    echo "@cycles: ", $cycles, PHP_EOL;
    echo "@length: ", $num, PHP_EOL;
    echo "@tries: ", $tries, PHP_EOL;

    $array = prepare($num);
    $key = uniqid('', true);

    echo "array_key_exists: ";
    $times = array();
    for ($j=0; $j<$tries; $j++) {
        $t = microtime(true);
        for($i = 0; $i < $cycles; $i++) {
            array_key_exists($key, $array);
        }
        $times[$j] = microtime(true) - $t;
    }
    printf('%0.4f second(s)'.PHP_EOL, array_sum($times)/$tries);

    echo "isset: ";
    $times = array();
    for ($j=0; $j<$tries; $j++) {
        $t = microtime(true);
        for($i = 0; $i < $cycles; $i++) {
            isset($array[$key]);
        }
        $times[$j] = microtime(true) - $t;
    }
    printf('%0.4f second(s)'.PHP_EOL, array_sum($times)/$tries);
}

$cycles = 1000000;
$tries = 5;

echo '----------------------', PHP_EOL;
test(1000, $cycles, $tries);
echo '----------------------', PHP_EOL;
test(10000, $cycles, $tries);
echo '----------------------', PHP_EOL;
test(100000, $cycles, $tries);
echo '----------------------', PHP_EOL;
test(1000000, $cycles, $tries);
echo '----------------------', PHP_EOL;