When I click for the redirect link it is showing the content of index.php instead of single_assignment_page.php

Question

When I click on a link to redirect to 'single_assignment_page.php' it is not showing the content of this page but it is showing the content of 'index.php'. That's something I don't want but in the URL bar it is showing the right URL that I want. I'm working with my own theme in WordPress.

front-page.php with the link at the bottom

<div class="container">
<h2 class="title-section" >Opdrachten</h2>
<p>Hieronder staan de opdrachten vermeld die zijn ingezonden door bedrijven en particulieren. Het is gesorteerd van nieuw naar oud. De opdrachten zijn alleen te zien voor ingelogde gebruikers.</p>

<?php
$results = $wpdb->get_results( "SELECT id, voornaam, achternaam, bedrijfsnaam, telefoonnummer, emailadres, titel_opdracht, opdrachtbeschrijving FROM form_entry"); // Query to fetch data from database table and storing in $results
if(!empty($results))                        // Checking if $results have some values or not
{
    echo "<table width='100%' border='0'>"; // Adding <table> and <tbody> tag outside foreach loop so that it wont create again and again
    echo "<tbody>";
    foreach($results as $row){
        echo "<tr>";                           // Adding rows of table inside foreach loop
        echo "<th>ID</th>" . "<td>" . $row->id . "</td>";
        echo "</tr>";
        echo "<td colspan='2'><hr size='1'></td>";
        echo "<tr>";
        echo "<th>Voornaam</th>" . "<td>" . $row->voornaam . "</td>";   //fetching data from user_ip field
        echo "</tr>";
        echo "<td colspan='2'><hr size='1'></td>";
        echo "<tr>";
        echo "<th>Achternaam</th>" . "<td>" . $row->achternaam . "</td>";
        echo "</tr>";
        echo "<td colspan='2'><hr size='1'></td>";
        echo "<tr>";
        echo "<th>Bedrijfsnaam</th>" . "<td>" . $row->bedrijfsnaam . "</td>";
        echo "</tr>";
        echo "<td colspan='2'><hr size='1'></td>";
        echo "<tr>";
        echo "<th>Telefoonnummer</th>" . "<td>" . $row->telefoonnummer . "</td>";
        echo "</tr>";
        echo "<td colspan='2'><hr size='1'></td>";
        echo "<tr>";
        echo "<th>E-mailadres</th>" . "<td>" . $row->emailadres . "</td>";
        echo "</tr>";
        echo "<td colspan='2'><hr size='1'></td>";
        echo "<tr>";
        echo "<th>Titel opdracht</th>" . "<td>" . $row->titel_opdracht . "</td>";
        echo "</tr>";
        echo "<td colspan='2'><hr size='1'></td>";
        echo "<tr>";
        echo "<th>Opdrachtbeschrijving</th>" . "<td>" . $row->opdrachtbeschrijving . "</td>";
        echo "</tr>";
        echo "<td colspan='2'><hr size='1'></td>";
        echo "<tr>";
        echo '<td><a href="single_assignment_page.php?id='.$row->id.'">Hello</td>';
        echo "</tr>";
    }
    echo "</tbody>";
    echo "</table>";

}
?>

single_assignment_page.php

<?php $assignment_id = $_GET['id']; ?>

<?php get_header(); ?>

<?php the_content(); ?>
<p>Hey</p>
<p>Hey</p>
<p>Hey</p>
<p>Hey</p>
<p>Hey</p>
<p>Hey</p>
<p>Hey</p>
<?php get_footer(); ?>

I already tried some things:

• Tried to redirect without the id at the end of the url, then he still shows the index.php content.
• Tried to redirect to a other PHP file, but it is still showing the content of index.php.
• Edit the .htaccess file but I still got the same problem.

I hope someone can help me out, because I've been stuck with this problem for a whole day. Thanks for the effort.