PHP / HTML在提交后记住选定的值

I know there are many questions ask about this one, but I tried a lot from here and for some reason, they did't work for me.

I populate the list from mysqli.:

<select multiple="multiple" name="formCountries[]" size="5" id="keuzeproduct">
  <option value="keuze1" disabled selected="selected[]" multiple="multiple">Selecteer Product</option>
  <?php
       $sql = "SELECT DISTINCT Id, Product FROM metingen group by Product order by Product";
       $resultset = mysqli_query($conn, $sql) or die("database error:". mysqli_error($conn));
       while( $rows = mysqli_fetch_assoc($resultset) ) {
          ?>
          <option value="<?php echo $rows["Product"];?>" ><?php echo $rows["Product"]; ?></option>
          <?php } ?>
       </select>

How can I get it so the selected one stays selected after submit?

Get the value that has been selected and pass it to $selected_product and check with every option you are adding. This should solve your case

   <select multiple="multiple" name="formCountries[]" size="5" id="keuzeproduct">
            <option value="keuze1" disabled selected="selected[]" multiple="multiple">Selecteer Product</option>
            <?php
                $selected_product = $your_value_here; //get the selected product from the database on load
                $is_selected  = "";
                $sql = "SELECT DISTINCT Id, Product FROM metingen group by Product order by Product";
                $resultset = mysqli_query($conn, $sql) or die("database error:". mysqli_error($conn));
                   while( $rows = mysqli_fetch_assoc($resultset) ) {
                      if($selected_product == $rows["Product"]){
                         $is_selected = "selected";
                      }
                    ?>
            <option value="<?php echo $rows["Product"];?>" <?php echo $is_selected; ?>><?php echo $rows["Product"]; ?></option>
                  <?php } ?>
        </select>

Wrap the option generation with a function:

function makeOption($value)
{
    echo "<option value=\"$value\"";
    if(isset($_POST['formCountries']) && in_array($value, $_POST['formCountries']))
        echo ' selected="selected"';
    echo ">$value</option>";
}

Use it in while loop:

while( $rows = mysqli_fetch_assoc($resultset) )
    makeOption($rows["Product"]);

Simpy you can do like this for the option script:

<option value="<?php echo $rows["Product"];?>" <?=$rows['Product']=='//your val'?"selected":""?>><?php echo $rows["Product"]; ?></option>
                  <?php } ?>