suggest that highlight_string should add callback params.

higlight_string($str, $html_block_processer, $php_function_processer);

$html_block_processer will be call to process each block of HTML code in $str (highlighting html/javascript code)

$php_function_processer will be call to process function name in $str (e.g.: adding link for functions)

or maybe: higlight_string($str, $html_block_processer, $php_document_baseurl);