Including files for templates

I'm trying to write a template system, my template is the HTML layout, and my content is fetched from another source. However I don't quite understand how to output the template so that all the variables are parsed by PHP. Simple version of what I'm trying to do; ob_start( ); extract( $params, EXTR_PREFIX_SAME, "am_"); $pageContent = file_get_contents( "page_to_display.html"); include( "template.html"); echo ob_get_clean( ); $pageTitle is in the template, it's replaced, $pageContent is in the template, it's replaced. But any variables within the page_to_display are simply output into the page rather than processed by PHP. I realise that file_get_contents is basically returning a string and I"m just doing string replacement when I include the template.html, so my only question is, what's the actual way of doing this? Thanks