Note that E_NOTICE will warn you about uninitialized variables, but assigning a key/value pair counts as initialization, and will not trigger any error :
<?php
error_reporting(E_ALL);
$foo = $bar; //notice : $bar uninitialized
$bar['foo'] = 'hello'; // no notice, although $bar itself has never been initialized (with "$bar = array()" for example)
$bar = array('foobar' => 'barfoo');
$foo = $bar['foobar'] // ok
$foo = $bar['nope'] // notice : no such index
?>