How to store multiple values from dropdown in php variable

Question

I am trying to get a table to generate based on the sql query in this page, and I have it working with one item in the dropdown menu.

However if I select two, nothing happens, probably because I am either storing the variable incorrectly in $forum or I am accessing it incorrectly in the if $row statement further down.

Please help me understand what to do.

Thanks!

<select name="forums" multiple>

    <option disabled selected value> -- Select a forum -- </option>
<?php
$con = mysqli_connect("host","user","pass","db");
        if (!$con) {
            die('Connection failed: ' . mysqli_connect_error() . '<br>');
            }
$productName='
SELECT p.name 
  FROM product as p 
  JOIN ownedproducts as o 
    on o.productID = p.productID 
 WHERE usersID = 2;
';
        $result=mysqli_query($con, $productName);
        while ($row = mysqli_fetch_assoc($result)) {
                  unset($id);
                  $id = $row['name'];
                  echo '<option value="'.$id.'">'.$id.'</option>';
                  }
        if(isset($_GET["forums"])){
       $forum=$_GET["forums"];
       echo "selected forum => ".$forum;
   }
?>
</select>
    <input type="submit" class = "submit" value="submit" onclick="this.form.submit();" />
    </form>
    </div>
<hr>
<table class = "table">
<tr id=table-row>
<th id=table-header>Name</th>
<th id=table-header>Company</th>
<th id=table-header>Type</th>
</tr>
<?php
while ($row = $query1->fetch()) 
{
    if ($row['name']== $forum) {
        echo "<tr id=" . $row['productID'] . ">";
        echo "<td>" . $row['name'] . "</td>";
        echo "<td>" . $row['company'] . "</td>";
        echo "<td>" . $row['Type'] . "</td>";
        echo "</tr>";
        }
}
?>```