使用php从多个表mysql中删除

I have literally tried everything i've found on the internet about deleting from multiple tables in same query, but it just ends up deleting from the first table(Posts)..

So how does one do this the correct way with php and pdo?

examples of what i've tried=

$dsn = "DELETE FROM Posts, Comments USING Posts, Comments WHERE Posts.ID = Comments.PostID  AND Comments.PostID=:my var";

$dsn = "DELETE FROM Posts LEFT JOIN Comments ON `Comments.PostID` = `Posts.ID` WHERE `Posts.ID`=:tit";

$dsn = "DELETE Posts , Comments  FROM Posts  INNER JOIN Comments  WHERE Posts.ID = Comments.PostID and Posts.ID =:myvar";

The tables looks as following:

 TABLE: Posts

    ID(PK AI) 
    Title(VARCHAR)
    Post(VARCHAR)
    Author(VARCHAR)
    Date(DATETIME)



TABLE: Comments

ID(PK AI so all comments get unique id's)
Name(VARCHAR)
Comment(VARCHAR)
Date(DATETIME
PostID(INT)

You have different ways to solve this:
1- delete with inner join

$dsn = "DELETE Posts.*, Comments.*  FROM Posts  INNER JOIN Comments  WHERE Posts.ID = Comments.PostID and Posts.ID =:myvar";

2- delete cascade, drop if exist foreing key and execute this:

ALTER TABLE Comments
  ADD CONSTRAINT fk_postid 
  FOREIGN KEY (PostID) 
  REFERENCES Posts(ID) 
  ON DELETE CASCADE;

3- using trigger after delete in the table Posts

  delimiter $$
    CREATE TRIGGER `after_delete_Posts`     
      AFTER DELETE ON `Posts`     
      FOR EACH ROW     
    BEGIN
      DELETE FROM Comments where PostID = OLD.id;
    END
    $$
    delimiter ;