Using strlen() in vBulletin

[Lays]

I am trying to do this:

<!-- START STRETCHY POSTBOX FOR LONG NAMES -->
			
			
<if condition="strlen($post[musername]) < 25">
<div id="postmenu_$post[postid]" style="white-space: nowrap;">
<else /> <div id="postmenu_$post[postid]">
</if>
			
			
<!-- END STRETCHY POSTBOX FOR LONG NAMES -->

How can I use the strlen so it works in vbulletin?