One can use question mark placeholders for out-parameters in the PREPARE statement. Only SELECT … INTO can be used this way:

prepare test from "select id into ? from t1 where val=?";
execute test using @out, @in;

This is particularly convenient when used with compound statements:

PREPARE stmt FROM "BEGIN NOT ATOMIC
  DECLARE v_res INT;
  SELECT COUNT(*) INTO v_res FROM t1;
  SELECT 'Hello World', v_res INTO ?,?;
END"|