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Use multiple files in a WordPress plugin – Call to undefined function add_action()

Solution:

In PHP include is a statement, not a function.

So it should be:

<?php
include dirname( __FILE__ ) .'/1.php';
include dirname( __FILE__ ) .'/2.php';
include dirname( __FILE__ ) .'/3.php';
include dirname( __FILE__ ) .'/4.php';
?>

Or to be perfect:

<?php
require_once dirname( __FILE__ ) .'/1.php';
require_once dirname( __FILE__ ) .'/2.php';
require_once dirname( __FILE__ ) .'/3.php';
require_once dirname( __FILE__ ) .'/4.php';
?>