I am new to sessions, and think I get the basics of them, they seem to act like containers holding information which you can use on at a later stage and are linked to your UID.

I am wondering how I would save the ID of a page (example ID123) on the click of a button, and what exact code I'd have in the header.

Much appreciated in advance!

EDIT:

This is on wordpress, sorry to add this, each page has an 'event ID' I want this to be stored for use later (at a kind of checkout page) sorry for not adding this!

In wordpress you cannot pass custom url parameters. So you cannot send something like http://yourwordpresswebsite.com/?custom_param='value'

WOrdpress doesnt allow it. SO if all you want is a Post ID or a Page ID. It is easy to grab in wordpress. Do:

$val= $post->ID

Thats it and $val is set. The way I do if I have to send any custom parameters to a different file is by sending it as a form variable. There might be better ways but I am new to Wordpress and php too. So this is what I do:

<?php
echo "<form action='php_file.php' method='post' name='form_name'>
<input type='hidden' name='eventid' value='$event_id' />
<input type='submit' name='submit' value='submit'/>
</form>";

?>

If you want this to be your session info you can just add it in your php file where you are collecting the above fields.

session_start();
$_SESSION['pageId'] = (int) $_GET['eventid'];

/*  Do not forget to check if the get variable is clean before you perform any operations, use mysql_real_escape_string(). It is a very important security measure. */

This might not be the best approach as I told you I am new to wordpress too.

Saving page ID within $_SESSION

session_start();
$_SESSION['pageId'] = (int) $_GET['ID'];

Now you can use $_SESSION['pageId'] to get its value where you need.

Make sure to put session_start(); at top of your script where you use session-related functions or $_SESSION array.

To unset it when you don't need it anymore, you would do:

unset($_SESSION['pageId']);
session_destroy();
session_regenerate_id();

update

On button click you would do something like this:

var btn = document.getElementById('btnId');
btn.onClick = function() {
  window.location = 'saveSession.php?id=xxx' // replace xxx with your id value
};

Now in saveSession.php you wll have to use code like shown above for storing it in session.

You can also use Ajax though.