ZCE Zend Certified Engineer String return problem

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function stringAppend(&$str)

{

        return $str = $str."append";

}

function stringPrepend(&$str)

{

         $str = "prepend".$str;

}

$aString = "PHP";

echo stringPrepend(stringAppend($aString));

Answer: NULL

becuase stringPrepend dint return anything