allow php in page

I have installed Allow PHP in Posts and Pages. i wrote
[php]
include ‘x.php’;
[/php]
in a hello page.
I want to call another php code from x.php to y.php
For this i created new y page and include y.php there.
But it is showing warning

Warning: session_start() [function.session-start]: Cannot send session cookie – headers already sent by (output started at /home/infoav/public_html/bookbazaar/wp-content/themes/twentythirteen/header.php:4) in /home/infoav/public_html/bookbazaar/wp-content/plugins/allow-php-in-posts-and-pages/profile.php on line 3

How to solve this problem
Please help

Tamanna

https://www.ads-software.com/plugins/allow-php-in-posts-and-pages/