Addition

PHP additions apply to two different types : numbers and arrays.

For numbers, aka integers or float, this is the classic mathematical addition.

For arrays, this is a special version of array merge : the arrays are merged, and keys are kept once they are written. This features only exists for addition and arrays. Substraction is not available.

<?php

$a = 1 + 2.3; // 3.3

$b = [1] + [3, 4]; // [1, 4]

?>

Documentation

See also Array operators, Combining arrays using + versus array_merge in PHP

Related : Array, integer, Floating Point Numbers