Backlinks pointing to a redirected PHP file

If i have some backlinks pointing to index.php and have index.php do a "header location" redirect to another page, will the page which it is being redirected to get the juice?

Use 301 redirect through .htaccess instead. This is what works for SEO.

You can do a 301 redirect via php:
<?php
Header( "HTTP/1.1 301 Moved Permanently" );
Header( "Location: destination" );
?>

Yes, all of the link juice will then point to that new page. There will some small loss (~5-15%) because you are redirecting the page, but basically all of this will be directed to that other page.