Neutral Element
A neutral element is a piece of data which has no effect with a particular operation.
For example, 0 is the neutral element for the addition, as 0 + $a is the same as $a + 0 and the same as $a, without any type change.
The neutral element for the multiplication is 1; the neutral element for the concatenation is the empty string ''.
It is usually possible to define a neutral element for any operation or method call. For objects, they are called null objects, as they could replace the null scalar value.
<?php
$a = 10;
$a *= 1; // this does nothing
$a += 1; // this does nothing
?>