PHP调用sqlserver存储过程并获取返回值

public function getConn($dbHost, $dbName, $dbUser, $dbPass) {
$dsn = 'sqlsrv:server='.$dbHost.';Database='.$dbName;
$conn= new PDO($dsn, $dbUser, $dbPass);
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
return $conn;
}

public function runStoredProcedure($dbHost, $dbName, $dbUser, $dbPass){
$conn = getConn($dbHost, $dbName, $dbUser, $dbPass);
$storedProcedureName = $runner->m_StoredProcedureName;
$query = "{:retval = CALL $storedProcedureName()}";
$stmt = $conn->prepare($query);
$stmt->bindParam('retval', $retval, PDO::PARAM_INT|PDO::PARAM_INPUT_OUTPUT, 4);
$stmt->execute();
return $retval;
}