Explanation in Shiflett's PHP Security Briefing

["afan@xxxxxxxx" <afan@xxxxxxxx>]

I was reading PHP Security Briefing from brainbulb.com (Chris Shiflett) 
and didn't get one thing:
in example:

<?php
   $clean = array();
   if (ctype_alnum($_POST['username']))
   {
       $clean['username'] = $_POST['username'];
   }
?>

why to set the $clean as array? what's wrong if I use:

<?php
   if (ctype_alnum($_POST['username']))
   {
       $clean = $_POST['username'];
   }
?>


-afan