include
Inclusion, often represented by the include keyword, is the process to read an external file, and evaluate with the current application for execution. It may define new structures, and execute code.
Inclusion relies on four language structures: include, require, include_once, require_once.
Inclusion is often hidden in autoload() functions: the code is included class per class, upon request.
vars.php
<?php
$color = 'green';
$fruit = 'apple';
?>
test.php
<?php
echo "A $color $fruit"; // A
include 'vars.php';
echo "A $color $fruit"; // A green apple
?>