Save database row Id in session for use later – SEO

I’m trying to save the row $row['Id'] in a php session so I can use this variable on the next page to have it fill the $product_id variable dynamically.

So far this is what I’ve got:

<?php $sql = "SELECT * FROM assortiment WHERE Categorie = '$productid' ORDER BY Id DESC ";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // output data of each row
    while($row = $result->fetch_assoc()) {
        SESSION_START();
        $identi = $row['Id'];
        $_SESSION["product_id"] = $identi;
        echo "<a href='/dbtest/dinner/amsterdam/".$row["Slug"]. "/" . $row["Id"] . "'><div class='products'><div class='col-sm-3'><div class='product-img'>";
        echo "<img src='http://www.example.nl/Uploaded_files/Thumbs/" .$row['Fotomateriaal']. ".jpg'>";
        echo "</div></div><div class='col-sm-6'>";
        echo "<div class='h2-container'><h2>" . $row["Product"]. "</h2></div>" .  $row["Samenvatting"]. "" . $_SESSION["product_id"] . "";
        echo "</div><div class='col-sm-3'><div class='col-sm-12 text-right'>
                    <div class='border'>
                        <p style='margin-bottom:20px;width:30px;'><i class='fa fa-heart' aria-hidden='true'></i></p>
                    </div>
                </div>";
        echo "<table>
                <tr>
                    <td><i class='fa fa-users' aria-hidden='true'></i></td>
                    <td><p>vanaf 10 personen</p></td>
                </tr>
                <tr>
                    <td><i class='fa fa-clock-o' aria-hidden='true'></i></td>
                    <td>" . $row["Tijdsduur"] .  " uur</td>
                </tr>
                <tr>
                    <td><i class='fa fa-euro' aria-hidden='true'></i></td>
                    <td>vanaf " . $row["VerkoopPP40"] ." p.p. <small>excl btw</small></td>
                </tr>
            </table></div></div></a>";
    }
} else {
    echo "0 results";
}?>

As you can see I’m trying to save the $row['Id']. First I make this ID in a $identi variable so that I can echo it. Then I save the ID in a $_session["product_id"] and echo it later on in the HTML. This works. But when I click on the link to go to the next page it looks like the session isn’t saved.

I cannot echo the product_id on the next page using:

<?php
    if(isset($_SESSION['product_id'])){
    echo $_SESSION['product_id'];
    };
?>

I think it’s not being stored, or (probably) I’m doing this all wrong? I found something on stack-overflow that started the session inside another part of the database query:

<?php 
    $sql = "SELECT * FROM assortiment WHERE Subcategorie = '$product_id'";
    $result = mysqli_query($conn, $sql);
    if (mysqli_num_rows($result) > 0) {
    while($row = mysqli_fetch_assoc($result)) {
        $fotos = $row["Fotomateriaal"];
        $productnaam = $row["Product"];
        $contenttext = $row["WebTekst"];
        $tijdsduur = $row["Tijdsduur"];
        $prijs = $row["VerkoopPP40"];
        $sub = $row["Subcategorie"];
        $cate = $row["Categorie"];
        $seo = $row["SEO_Pagetitle"];
        $youtube = $row["Actief_Avontuur"];
        $_SESSION["product_id"] = $row["Id"];
            }
} else {
    $value = "Geen resultaten";
}       ?>

But this give me nothing also. Does anyone know why this isn’t working. Normally everything works fine if I want to store a variable in a session like normal (just using PHP and WordPress) but this just wont work. Any help would be much appreciated.