$sql = "INSERT INTO league(user,email,password,rival,key) VALUES ('$this->user','$this->email','$this->pass','0','0')";
mistake:
You have an error in your SQL syntax; check the syntax for the use of the syntax to use the 'key') VALUES ('user1', 'user1@gmail.com', 'usrpsw', '0', '0') 'at line 1
field key int (11) NULL (no) DEFAULT 0