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<!DOCTYPE html>
<html>
<body>
<?php
$a = 0;
echo "a is " . is_null($a) . "<br>";
$b = null;
echo "b is " . is_null($b) . "<br>";
$c = "null";
echo "c is " . is_null($c) . "<br>";
$d = NULL;
echo "d is " . is_null($d) . "<br>";
?>
</body>
</html>