I think you may want to take a different approach to your problem like other people have mentioned.
I'm going to presume your two files are in the same directory, if not you may need to alter the below slightly.
source.php
<?php
// The variable we want to work with
$multiplicator = 2;
index.php
<?php
require 'source.php';
echo (1 * $multiplicator) . ' Apples';
I'm not exactly sure on what your use case is for this to be honest, it seems a little odd but the above should at least fix your issue.
The reason why your current code is not working is due to the fact file_get_contents() returns the content as a string, that string isn't parsed by PHP so you do not have access to any variables you declared in a file that you call by file_get_contents()
Hope that helps, and here's the DOCs for file_get_contents() and require:
http://php.net/manual/en/function.file-get-contents.php
http://php.net/manual/en/function.require.php
includeorrequirethesource.phpfile so it's contents get parsed as php and not just a string.