how to implement previous and next in ussd code example

Example: ussd php

<?php// Reads the variables sent via POST$sessionId   = $_POST["sessionId"];  $serviceCode = $_POST["serviceCode"];  $text = $_POST["text"];//This is the first menu screenif ( $text == "" ) {$response  = "CON Hi welcome, I can help you with Event Reservation \n";$response .= "1. Enter 1 to continue";}// Menu for a user who selects '1' from the first menu// Will be brought to this second menu screenelse if ($text == "1") {$response  = "CON  Pick a table for reservation below \n";$response .= "1. Table for 2 \n";$response .= "2. Table for 4 \n";$response .= "3. Table for 6 \n";$response .= "4. Table for 8 \n";}//Menu for a user who selects '1' from the second menu above// Will be brought to this third menu screenelse if ($text == "1*1") {$response = "CON You are about to book a table for 2 \n";$response .= "Please Enter 1 to confirm \n";}else if ($text == "1*1*1") {$response = "CON Table for 2 cost -N- 50,000.00 \n";$response .= "Enter 1 to continue \n";$response .= "Enter 0 to cancel";}else if ($text == "1*1*1*1") {$response = "END Your Table reservation for 2 has been booked";}else if ($text == "1*1*1*0") {$response = "END Your Table reservation for 2 has been canceled";}// Menu for a user who selects "2" from the second menu above// Will be brought to this fourth menu screenelse if ($text == "1*2") {$response = "CON You are about to book a table for 4 \n";$response .= "Please Enter 1 to confirm \n";}// Menu for a user who selects "1" from the fourth menu screenelse if ($text == "1*2*1") {$response = "CON Table for 4 cost -N- 150,000.00 \n";$response .= "Enter 1 to continue \n";$response .= "Enter 0 to cancel";}else if ($text == "1*2*1*1") {$response = "END Your Table reservation for 4 has been booked";}else if ($text == "1*2*1*0") {$response = "END Your Table reservation for 4 has been canceled";}// Menu for a user who enters "3" from the second menu above// Will be brought to this fifth menu screenelse if ($text == "1*3") {$response = "CON You are about to book a table for 6 \n";$response .= "Please Enter 1 to confirm \n";}// Menu for a user who enters "1" from the fifth menuelse if ($text == "1*3*1") {$response = "CON Table for 6 cost -N- 250,000.00 \n";$response .= "Enter 1 to continue \n";$response .= "Enter 0 to cancel";}else if ($text == "1*3*1*1") {$response = "END Your Table reservation for 6 has been booked";}else if ($text == "1*3*1*0") {$response = "END Your Table reservation for 6 has been canceled";}// Menu for a user who enters "4" from the second menu above// Will be brought to this sixth menu screenelse if ($text == "1*4") {$response = "CON You are about to book a table for 8 \n";$response .= "Please Enter 1 to confirm \n";}// Menu for a user who enters "1" from the sixth menuelse if ($text == "1*4*1") {$response = "CON Table for 8 cost -N- 250,000.00 \n";$response .= "Enter 1 to continue \n";$response .= "Enter 0 to cancel";}else if ($text == "1*4*1*1") {$response = "END Your Table reservation for 8 has been booked";}else if ($text == "1*4*1*0") {$response = "END Your Table reservation for 8 has been canceled";}//echo responseheader('Content-type: text/plain');echo $response?>