Set previously selected value in Dropdown box using PHP

So, what im trying to achieve is that after the user hits submit, the dropdown retains the value that the user have chosen. This is my code for the dropdown, i do understand that i have to selected = 'selected', but i couldn't figure out how it fits in my code.

    if (mysqli_num_rows($result) > 0) {
      // output data of each row
     while($row_branch = mysqli_fetch_array($result)) {
         $menu_branch .= "<option value='".$row_branch["0"]."'>" . 
         $row_branch["0"]. "</option>";
      }
    }

    echo $menu_branch;