title: Allow only English letters and numbers in php
body: <p>I'm trying to create a filter to allow users to use only English letters (Lowercase &amp; uppercase) and numbers. how can I do that? (ANSI)
  (not trying to sanitize, only to tell if a string contain non-english letters)
  That filter should get me a clean database with only english usernames, without multibyte and UTF-8 characters.</p>
  
  <p>And can anyone explain to me why echo strlen(À) outputs '2'? it means two bytes right? wans't UTF-8 chars supposed to contain a single byte?</p>
  
  <p>Thanks</p>