Null
null is a special data type that represents a variable with no value or an undefined value. It is used to indicate the absence of a value. In other words, when a variable is assigned the value null, it means that the variable exists but has no valid data.
null may be used as a default value for variables, parameter or properties that are expected to hold some value later but don’t have a value assigned initially. It was also be used to unset a variable and free up memory, when using the operator (unset).
There is also a design pattern called Null Pattern, although its main goal is to remove usage of null and replace it with an actual object.
<?php
$a = null;
$B = NULL;
$c = \null;
// same as null|string $s
function trumpet(?string $s = null) {}
?>