Search function doesn't show anything - php

Question

I'm trying for a search function, I made a public function in a file called Topic, and I tried calling the method in the index.php, however it doesn't show anything, I think the problem lies with the while statement, but i'm not entirely sure.
Topic.php
class Topic{
public function searchPosts($postTags)
{
$conn = Db::getInstance();
$statementSearch = $conn->prepare("SELECT * FROM topics INNER JOIN posttopic ON topics.topicID=posttopic.topicID INNER JOIN posts ON posttopic.postID=posts.postID WHERE naam = :naam");
$statementSearch->bindValue(":naam", $postTags);
return $statementSearch->execute(array());
}
}
Index.php
spl_autoload_register(function ($class) {
include_once("classes/" . $class . ".php");
});
$_SESSION['KEYWORD'] = array();
$postArray = array();
//$allResults2 = array();
if (isset($_POST['Find'])) {
if (!empty($_POST['Find'])) {
$searchTopic = new Topic();
$postTags = $_POST['naam'];
$searchTopic->searchPosts($postTags);
while ($row = $statementSearch->fetch(PDO::FETCH_ASSOC)) {
$_SESSION['KEYWORD'][] = $row['postImageUrl'];
$postArray[] = $row['postID'];
}
}
if (count($_SESSION['KEYWORD']) === 0) {
$error = "Sorry no results!";
}
}
This is the html where it should be printed.
<?php
foreach (array_combine($_SESSION['KEYWORD'], $postArray) as $imageLink => $i) {
echo "<a href='./pin.php?postid=$i' ><img src='" . $imageLink . "'</a>";
}
?>