how to keep a session alive for 30 minutes and then destroy it in php

session.gc_maxlifetime
session.gc_maxlifetime specifies the number of seconds after which data will be seen as 'garbage' and cleaned up. Garbage collection occurs during session start.

But the garbage collector is only started with a probability of session.gc_probability divided by session.gc_divisor. And using the default values for that options (1 and 100 respectively), the chance is only at 1%.
Well, you could argue to simply adjust these values so that the garbage collector is started more often. But when the garbage collector is started, it will check the validity for every registered session. And that is cost-intensive.
Furthermore, when using PHP’s default session save handler files, the session data is stored in files in a path specified in session.save_path. With that session handler the age of the session data is calculated on the file’s last modification date and not the last access date:

Note: If you are using the default file-based session handler, your filesystem must keep track of access times (atime). Windows FAT does not so you will have to come up with another way to handle garbage collecting your session if you are stuck with a FAT filesystem or any other filesystem where atime tracking is not available. Since PHP 4.2.3 it has used mtime (modified date) instead of atime. So, you won't have problems with filesystems where atime tracking is not available.

So it additionally might occur that a session data file is deleted while the session itself is still considered as valid because the session data was not updated recently.
And second:

session.cookie_lifetime
session.cookie_lifetime specifies the lifetime of the cookie in seconds which is sent to the browser. […]