PHP and MySQLi close()

Question

I am new to MySQL and PHP and am attempting to make my own CMS to help make managing my websites easier. Can someone explain mysqli's close() function?

1. Is it necessary?
2. What exactly does it do?
3. I heard that after PHP runs its script that it closes the connection, is that true?
4. Lastly, is there a security issue when not closing your connection to the database?

Answer 1

You need to close the Mysql session when you manual set variables @@session or non-defined (set to @@session).
for example

 $mysqli->query("SET @uuid=UUID()");

https://dev.mysql.com/doc/refman/5.1/en/set-statement.html

Answer 2

Is it necessary?

No, PHP will end your connection after it finishes running.

What exactly does it do?

The reverse of mysqli_connect() -- it closes the active DB connection.

I heard that after PHP runs its script that it closes the connection, is that true?

Yes, see the answer to "Is it necessary?"

Lastly, is there a security issue when not closing your connection to the database?

Nope, no security issue. The connection can't be hijacked by an outsider or anything like that.

However, since the number of total connections available is limited, freeing the resource the second you're done with it is considered polite to close it when you're done. This is likely why you've been told to close it when you're done.