I think GordPocock may have misunderstood. The function
tpl_searchform(); doesn't do on a wiki page but in the template: /lib/tpl/prsnl10/main.php. For instance:
<div id="tmpl_header">
<div id="tmpl_header_left">
<?php
tpl_searchform();
In the above you don't need
<?php tpl_searchform(); ?> because the function is already in a php block. I know this works because I have tested it.