In PHP, what is the difference between checking for NULL using != null and === NULL, and when should each be used?

When checking for NULL in PHP, using != null will check for a value that is not NULL, while using === NULL will specifically check for a NULL value. It is recommended to use === NULL when you want to ensure that the variable is explicitly NULL, as using != null may also return true for other falsey values like 0 or an empty string.

// Using === NULL to check for NULL value
if ($variable === NULL) {
    echo "Variable is NULL";
}

// Using != null to check for non-NULL value
if ($variable != NULL) {
    echo "Variable is not NULL";
}