nullb.php

<?php

$var = null;

$start1 = microtime(true);
$i = 0;
while($i < 10000000){
    $ret1 = $var === null;
    $i++;
}
$end1 = microtime(true);

$start2 = microtime(true);
$j = 0;
while($j < 10000000){
    $ret2 = is_null($var);
    $j++;
}
$end2 = microtime(true);

echo('PHP Version: ' . PHP_VERSION . PHP_EOL);
echo('=== null' . PHP_EOL);
echo($end1 - $start1 . PHP_EOL);
echo(PHP_EOL);
echo('is_null' . PHP_EOL);
echo($end2 - $start2 . PHP_EOL);

results.txt

$ php ./null.php
PHP Version: 5.5.12
=== null
1.0364179611206

is_null
7.9186248779297

$ hhvm ./null.php
PHP Version: 5.5.99-hiphop
=== null
0.47122597694397

is_null
0.46273303031921