Characters in a string

From: CyberDog (milan_at_cdnforum.com)
Date: 03/29/05 

Date: Tue, 29 Mar 2005 08:49:51 +0200

Hello!

Does anyone know how to find out how many aplhanum chars are in a
variable, I need the function to limit, for an example, the length of
ones user name.

I tried to search the manual but I get an 403, must be something wrong
with my local mirror of php.net :(

Thank you!