Passing $post data in php

This may be hard to follow.
I need to pass data from page to page.  Follow me on this:
Page 1: Keyin data for main record (this is where the 1st $post data comes from)
Page 2: If  certain check box is "N", go to page 5.
    else
    Page 3:  Keyin more data.
    Page 4:  Write record and save new values for original record
Page 5: Write/Update main record

 I need to keep the $_POST data but it does not seem to work.
On page #2 when the check box is "Y", I tried
                $work = $_POST['Add_Name_1'];
                $_POST['Add_Name_1'] = $work;


But when I get to page #3, I get
Notice: Undefined index: Add_Name_1 in /home/langsyst/public_html/lansco/Contact_Insert.php on line 12
Line 12 is :
            echo "Post 1 = ".$_POST['Add_Name_1']."<br>";      

Thanks