calling into wordpress using php includes?

i’m attempting to do a very basic php includes call from an alternate page:
<?php include(“index.php”); ?>

but am receiving the following error:
Warning: Cannot modify header information – headers already sent by (output started at /home/kylemar1/public_html/press/test1.php:9) in /home/kylemar1/public_html/press/wp-includes/pluggable.php on line 868

any ideas? are there issues calling wordpress into another page via php?