I. What is null

Null is a special data type that presents a variable with no value.

A variable is null if it’s assigned a constant null or it’s not defined yet.

$a = null;
echo $a;
var_dump($a); // NULL
var_dump($b); // NULL because it's undefined.

II. Check if a variable is null

We can use the function is_null() to check if a variable is null.

$a = null;
var_dump(is_null($a)); // bool(true)

Or we can also use a triple equal sign comparison operator.

$a = null;
var_dump($a === null); // bool(true)

III. Casting

When null is cast into a string, it will be converted into an empty string.

It will be 0, false, and an empty array if we cast null into an integer, a boolean, or an array.

$a = null;
var_dump((string)$a); // string(0) ""
var_dump((int)$a); // int(0)
var_dump((bool)$a); // bool(false)
var_dump((array)$a); // array(0) { }