$_GET to store a dynamic session - php

Question

I'm a PHP noob and I need some help if you can.
I have a little website similar to a survey in witch i click on a link and in the next page I have get the previous variable and store it through all the survey.
In the first page I have 5 buttons to choose:
in the second page I use $_GET to have the first variable
<?php
// Starting session
session_start();
if (isset($_GET['q1'])){
$question_1 = $_GET['q1'];
$_SESSION['q1'] = $question_1;
echo '<br>';
echo $_SESSION['q1'];
}
else{
header('location: index.php');
exit();
}
?>
My problem is that is someone change the url (question-2.php?q1=ANYTHING-ELSE)
I need to redirect them to the home page.
Basically I need to have an If statement in witch I only echo the five results of the 5 href
So I can have one session $_SESSION['q1'] with dynamic content but restricted to this five.
I know it's wrong but i need to restrict the get to this:
if (isset($_GET['q1']) && (($_GET['q1'])= '1' or '2' or '3' or '4' or '5'){}
Thank you!

Answer 1

What you can do here is have an array of allowed values and check if q1 value is one of the values in the given array:
<?php
// Starting session
session_start();
$allowed = array(1, 2, 3, 4, 5);
if (isset($_GET['q1'])) {
$question = (int) $_GET['q1'];
if(in_array($question, $allowed, true)){
$_SESSION['q1'] = $question;
echo '<br>';
echo $_SESSION['q1'];
} else {
header('Location: index.php');
exit();
}
} else {
header('Location: index.php');
exit();
}