--- acl.xml ---
<files>
   <acl file="index.php">
      <user name="Lord Valentine">
         <read>1</read>
         <write>1</write>
      </user>
    </acl>
</files>


--- php ---

function checkPerm($user,$file) {
   $acl = simplexml_load_file('acl.xml');
   if (($user = $acl->xpath("//acl[@file='$file']/user[@name='$user'])"))) {
      if ($user->read == 1) { do_whatever(); }
   }
}