Pass value from one PHP file to another

I'm trying to pass a value which is input into a box on one Php page (itinerary.php) into another Php page ('submit.php') so it can, from there, be saved into a database. But I can't quite figure out how to get it across. I've tried using GET as you can see from code below, but I am already using a GET statement to receive and acknowledge another value from that very same page 'submit'. I guess I am overcomplicating it, but my knowledge of Php is still pretty limited at this stage so any ideas would be appreciated!

This is an extract from the itinerary.php file (it sits within a Bootstrap/Html framework. Note the entry which contains the input box for the sequence number).

<h3><br>YOUR ITINERARY</h3> <?php //Display contents of itinerary if(!empty($_SESSION['itinerary'])){ //Retrieve details of each location in array from database $query = "SELECT * FROM locations WHERE loc_id IN ("; foreach ($_SESSION['itinerary'] as $loc_id=>$value) {$query.=$loc_id.',';} $query = substr($query, 0, -1).')ORDER BY loc_id ASC'; $result = mysqli_query($db, $query); echo'<table><tr><th colspan="5">LOCATIONS IN YOUR ITINERARY</th></tr>'; //Display locations in array while ($row = mysqli_fetch_array($result, MYSQLI_ASSOC)){ $loc_id = $row['loc_id']; echo"<br><td>{$row['loc_name']}</td></tr><br> <td>{$row['loc_desc']}</td> <td><p>Seq. No</p><input type=\"text\" size=\"3\" name=\"sequence\" value=????????>NOT SURE WHAT TO ADD INTO THIS LINE TO RETAIN THE INFO THAT IS INPUT</td> <td><a href=remove_loc.php?value=$loc_id>Remove location</a><br></br></td> </tr><br></table>"; }//while print_r($_SESSION); mysqli_close($db); }//if else {echo '<p><br>Your itinerary is empty.<br></p>';} echo '<br><p> <a href=submit.php?submit>Save itinerary</a> <a href=clear_itin.php?clear>Clear itinerary</a> <a href="user_home.php">Your details</a> <a href="logout.php">Logout</a> </p>'; ?> 

And this is where I am trying to receive it and then use it in a SQL command to add to the database. (Again, this is within a Bootstrap framework)You can ignore the first SQL Insert statement as it is passing other info in successfully anyway..

<div class="row"> <div class="col-md-9"> <?php if (isset($_GET['sequence'])){ $sequence = $_GET['sequence']; }//if if (isset($_GET['submit'])){ $query = "INSERT INTO itineraries (user_id, date_created) VALUES (".$_SESSION['user_id'].", NOW())"; $result = mysqli_query($db, $query); $itinerary_id = mysqli_insert_id($db); $retrieve_locs = "SELECT * FROM locations WHERE loc_id IN ("; foreach ($_SESSION['itinerary'] as $id=>$value) {$retrieve_locs.=$id.',';} $retrieve_locs = substr($retrieve_locs, 0, -1).')ORDER BY loc_id ASC'; $result = mysqli_query($db, $retrieve_locs); //Store items in itin_locs db while ($row = mysqli_fetch_array ($result, MYSQLI_ASSOC)){ //This is the command I have been trying to use, commented out. The second one below this works fine, but obv doesn't input a sequence number. //$insert_locs = "INSERT INTO itin_loc (itinerary_id, loc_id, sequenceNo) VALUES ($itinerary_id, ".$row['loc_id'].", $sequence)"; $insert_locs = "INSERT INTO itin_loc (itinerary_id, loc_id) VALUES ($itinerary_id, ".$row['loc_id'].")"; $insert_result = mysqli_query($db, $insert_locs); echo mysqli_error($db); if ($insert_result === FALSE) { die("Query failed!" . mysql_error() . $insert_locs);} }//while mysqli_close($db); echo"<p>Your itinerary is saved!. Itinerary number is #".$itinerary_id."</p><br>"; $_SESSION['itinerary']= NULL; echo '<p> <a href="user_home.php">Your details</a> <a href="logout.php">Logout</a> </p>'; }//if else {echo '<p>Your itinerary is empty.<br></br></p>';} ?>