It's best to 'quote' your 'keys' and here's why:
define('a','b');
$arr = array('a' => 'apple', 'b' => 'banana');
print $arr[a]; // banana
print $arr['a']; // apple
In otherwords, PHP first looks for a constant (in our example, a constant named a), if PHP does not find such a constant it'll then throw an error of level E_NOTICE, then look for the key named 'a' in the given array.
There is a time when using quotes is bad, for example:
print "I'm in a string so don't do this $okay['foo'] otherwise you'll get a parse error";
print "I'm {$okay['though']} and $me[too] because we're in strings";
See?
