Spent all day banging my head against a brick wall.
Tried to use INOUT or OUT and getting the return variable into PHP using Mysql v5.5.16 on XAMPP.
"MySQL doesn't supporting binding output parameters via its C API. You must use SQL level variables:"
<?php
$stm = $db->prepare("CALL sp_mysp(:Name, :Email, @sp_result)");
$outputArray = $db->query("select @sp_result")->fetch(PDO::FETCH_ASSOC);
?>
So the 'workaround' for Mysql and PDO is to use two SQL calls.
Hope this helps someone.