How to convert a checkbox into a dropdown-checkbox?

I have an html table with a checkbox list on one of the columns. I'd like for the checkbox to be inside a dropdown. I am looking to use only php and no JS or Ajax to do so.

echo"
               <td>
                    <form method='post'>
                        Select user roles<br/>
                        <input type='hidden' name='user_id' value=".$row["user_id"].">
";

                for ($a = 1; $a <= $count ; $a++){
                    $new_sql = "SELECT role_id from users_roles, users WHERE users_roles.user_id = '".$row["user_id"]."' AND users_roles.role_id = '" . $a . "' GROUP BY users_roles.user_id";

                    $checked_or_not = mysqli_query($connect, $new_sql);
                    if ($checked_or_not->num_rows != 0){
                        echo "<input type='checkbox' checked name='techno[]' value='$a' />" .$roles[($a-1)]. "<br>";
                    }

                    else if ($checked_or_not->num_rows == 0){
                        echo "<input type='checkbox' name='techno[]' value='$a' />" .$roles[($a-1)]. "<br>";
                    }

                }
                echo "
                    <button class='btn btn-primary' type='submit' name='checkSubmit' >Submit</button>
                    </form>
                </td>
";