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.