Description:
------------
In the documentation, there is no reference to a '_' being 
concatenated between the prefix and the variable name being 
extracted.  Should make this fact more clear in documentation 
to aleviate some problems such as illustrated below:

Reproduce code:
---------------
<?php
extract(array('test' => 123), EXTR_PREFIX_ALL, 'prefix');
echo $prefixtest;
?>

Expected result:
----------------
123

Actual result:
--------------
undefined variable $prefixtest (it's really $prefix_test)