As long as you have apache you can use that trick its htaccess
So in theory
<Files admin.php>
Order Deny,Allow
Deny from all
Allow from YOUR.IP.HERE.
</Files>
What this file says is that any source will be denied access, then it checks if you are that IP.
Problem you find is if you have a dynamic IP your been denied when it changes unless you change the file
The files part indicates what files this is applied to.