تابع ‎is_string()‎‎ در PHP

<?php
$a = "Hello";
echo "a is " . is_string($a) . "<br>";

$b = 0;
echo "b is  " . is_string($b) . "<br>";

$c = 32;
echo "c is " . is_string($c) .  "<br>";

$d = "32";
echo "d is " . is_string($d) . "<br>";

$e =  true;
echo "e is " . is_string($e) . "<br>";

$f = "null";
echo "f  is " . is_string($f) . "<br>";

$g = "";
echo "g is " . is_string($g)  . "<br>";
?>