Sessions aren’t enabled in wordpress by default, if you want to activate php sessions add this at the beginning of your functions.php
:
if (!session_id()) {
session_start();
}
You now can use $_SESSION['your-var'] = 'your-value';
to set a session variable. Take a look at the PHP documentation on sessions.
Update:
There was a second answer, which, in my mind, had a value too – unfortunately it got deleted, I’m re-adding the information here. The answer was referring to WP Session Manager a plugin written by @eamann as a alternative solution.
I read some things about the plugin, but never used it because – so far – I’m sticking with PHP sessions – never had the problem that I couldn’t use them. This is a statement/comment by the plugin author himself I found about some possible advantages.