Adding includes to files

I need to add PHP calls to include a file to each page as it is generated, the only thing is I can't get the includes to come through correctly: <! Code> $display_block .= "<?php include(\"nav/top_nav.html\"); ?>"; $display_block .= "<?php include(\"nav/side_nav.html\"); ?>"; <! End Code> At the end of the generation I write $display_block to a file as $display_block holds all the data it accumulates through the run. How do I rewrite the inlclude to work, or do I need to find another way to make the includes? The includes are all template files that I used on the site to keep everything consistent. Thanks, Robert