Needless to say you should find another solution ASAP. In the meantime you can eval the code like this:
$str="<h1>Welcome</h1><?php echo $motto?><br/>"; // Your DB content
eval("?> $str <?php ");
Demo: http://codepad.org/ao2PPHN7
I can’t stress that enough: eval is dangerous, and application code shouldn’t be in the database. Try a template parser like Smarty, Dwoo, or my favorite: Twig.