تابع ‎is_null()‎‎ در PHP

<?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>";
?>
<?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>";
?>اجرای کد