MySQL database connection not closed: what will happen?

Question

I am using PHP to query the MySQL database on my website. Please answer the following questions:

1. What will happen if I don't use mysql_close() when I am done with querying the database in the end? The connection will remain open? If yes then upto how much time? If no then why?
2. If I open and close a connection to MySQL at several places in a webpage, how is the performance affected? i.e. connection is made again everytime some access to database is required on a single webpage.
3. How is mysql_close() related to performance? Should I open a new connection everytime some access to database is required OR should I keep only one connection and close it in the end?
4. If I don't close the connection, then if the user is trying to access some data again, will the new connection be used or the old open connection will be used?

Answer 1

1. It will automatically close when the PHP script is done running during destruct phase.
2. Performance will negatively be affected. Opening a new socket (especially to an external database server) is more expensive and time consuming than just keeping a pointer to the current connection in memory.
3. See answer no. 2
4. The access to the data will be performed by a new PHP request. Hence, you will have a new database connection. No problems there.

I'd advise to open your database connection during construct phase, re-use that connection during the entire execution of your script (if it's OO based, assign a class variable for your database connection and use $this->db during the entire script), and close it during destruction (or don't bother at all closing it, as it will be closed anyway, even when not declared specifically).