is_null($x) vs $x === null in PHP

gistfile1.txt

<?php

error_reporting(E_USER_NOTICE);


//checking with ===
$a = array();
$time = microtime(true);
for($i=0;$i<10000;$i++) {
    if($a[$i] === null) {
        //do nothing
    }
}
echo 'Testing with === ', microtime(true) - $time, "\n";

//checking with is_null()
$time = microtime(true);
for($i=0;$i<10000;$i++) {
    if(is_null($a[$i])) {
        //do nothing
    }
}
echo 'Testing with is_null() ', microtime(true) - $time, "\n";


//checking with negation
$time = microtime(true);
for($i=0;$i<10000;$i++) {
    if(!$a[$i]) {
        //do nothing
    }
}
echo 'Testing with negation ', microtime(true) - $time, "\n";

//checking with instanceof
$time = microtime(true);
for($i=0;$i<10000;$i++) {
    if($a[$i] instanceof stdClass) {
        //do nothing
    }
}
echo 'Testing with instanceof ', microtime(true) - $time, "\n";

hmmm... ???

Testing with === 0.0075018405914307
Testing with is_null() 0.12902498245239
Testing with negation 0.0070021152496338
Testing with instanceof 0.0075008869171143