A quick test in 2022 on PHP 8.1 confirms there is still no need to micro-optimize NULL checks:
<?php
// Comparison Operator
$before = microtime(true);
$var = null;
for ($i=0 ; $i<1000000000 ; $i++) {
if($var === null) {}
}
$after = microtime(true);
echo ' ===: ' . ($after - $before) . " seconds\n";
// Function
$before = microtime(true);
$var = null;
for ($i=0 ; $i<1000000000 ; $i++) {
if(is_null($var)) {}
}
$after = microtime(true);
echo 'is_null: ' . ($after - $before) . " seconds\n";
// ===: 4.1487579345703 seconds
// is_null: 4.1316878795624 seconds