Showing records based on topic - php

Question

I got a problem with showing things when pressing on a topic.
What I want
I want too show the records out of the second database that has the correct topic_id so if you click on a topic it needs to only show the "onderwerpen"(dutch for subjects) that have that topic_id
My code:
<div class="box box-info">
<div class="panel panel-default">
<div class="panel-heading main-color-bg">
<h3 class="panel-title">Topics</h3>
</div>
<div class="panel-body">
<?php
$toppics = $app->get_topics();
foreach($toppics as $topic){
echo '<div class="col-md-6">';
echo '<div class="well dash-box">';
echo '<h3>'.$topic['onderwerp'].'</h3><br>';
echo '' .$topic['omschrijving'].'';
echo '</div>';
echo '</div>';
}
?>
</div>
</div>
</div><!-- /.box -->
Function I use:
public function get_topics(){
$getTopic = $this->database->query("SELECT * FROM topics ORDER BY id DESC");
$topics = $this->database->resultset();
return $topics;
}
The topic database
The onderwerpen database
After clicking on a subject It now shows all the "onderwerpen" out of the database
The code for that:
<?php
$onderrwerp = $app->get_onderwerpen();
foreach($onderrwerp as $onderwerp){
echo '<div class="well well-sm">';
if(file_exists('assets/images/profielfotos/'.$onderwerp['klant_id'])) {
echo '<img class="img-circle" src="/assets/images/profielfotos/'.$onderwerp['klant_id'].'/'.$onderwerp['foto'].'" />';
} else {
echo '<i style="font-size: 2.5em;" class="fas fa-user-circle"></i>';
}
echo '<b> '.$onderwerp['onderwerpnaam'].'</b>';
echo ' - ' . $onderwerp['voornaam'] . ' ' . $onderwerp['achternaam'];
echo '</div>';
}
?>
Get_onderwerpen function:
public function get_onderwerpen(){
$getOnderwerp = $this->database->query("
SELECT onderwerpen.*, klanten.foto, klanten.voornaam, klanten.achternaam FROM onderwerpen
LEFT JOIN klanten ON onderwerpen.klant_id=klanten.id
ORDER BY id ASC");
$onderwerpen = $this->database->resultset();
return $onderwerpen;
}