The very first line of the Variables section of the manual:

  Variables in PHP are represented by a dollar sign followed by the name of the 
  variable. The variable name is case-sensitive. 

It is at: http://www.php.net/manual/en/language.variables.php

A quick test can often help you figure these types of things out:

<?php
$bob['fred'] = 'One';
$bob['Fred'] = 'Two';
echo $bob['fred'] . "\n" . $bob['Fred'];
?>


Hope this helps,

Torben