Description:
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I have currently tested this on PHP 5.3.0 on a Windows XP environment.
Although it is unlikely that this type of data will be from user input but 
consider the test script.
When an undefined index notice occurs the index itself is not sanitized allowing 
for XSS type attacks.
Again it is very unlikely that this should come from user input but it is 
possible.

Test script:
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error_reporting(E_ALL | E_STRICT);
$a = array();
echo $a['<script>alert("A")</script>'];

Expected result:
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Notice: Undefined index: <script>alert("A")</script> in ...file...

Actual result:
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Notice: Undefined index: in ...file...

And a script alert popup.