PDO lastInsertId returning 0 (zero)

PDO::lastInsertId() should return the id of the last inserted row into the database. But sometimes it might return 0. Following are the reasons that could cause this:

• Last executed query was not an insert query
• The query statement was not executed with PDOStatement::execute()
• There is a semicolon at the end of the query

Basically, the first is the real cause, and others are just variations.