How to POST empty <select .. multiple> HTML elements if empty?

问题

I have the following multi-select box in a HTML form, where user can select one or more option.

<select id="eng_0" name="eng_0[]" multiple size="3">
  <option value="Privilégier">Privilégier</option>
  <option value="Accepté">Accepté</option>
  <option value="Temporaire">Temporaire</option>
</select>

When the user selects no option, the form is POSTed to a PHP backend but it creates no empty array value for $_POST['eng_0'] as if the field was not even on the form.

This is kind of like the unchecked checkbox that is not submitted problem.

Is there any way to have it POST the select object even if there is no selected option? It can be in jQuery if that helps.

回答1:

If your form is generated dynamically, you could include a hidden form element with the same name that contains a dummy value. Then, just ignore the dummy value, if the value you get for that variable is ['dummy_value'] then you can treat that as meaning "nothing selected" in your code.

回答2:

Is there a reason you can't treat the situation where the array isn't set as if it was sent with no contents?

if (!isset($_POST['eng_0']))
    $_POST['eng_0'] = array();

EDIT:

Add a hidden field whenever the multiple select is present in your form:

<input type="hidden" name="eng_0_exists" value="1"/>

Then check:

if (!isset($_POST['eng_0']) && isset($_POST['eng_0_exists']))
    $_POST['eng_0'] = array();

回答3:

If you add a hidden input before the multiple select element, it will send the hidden input value if none of the multiple select items have been selected. As soon as you select an option though, that selected value is used instead.

This way I was able to distinguish 2 different scenarios in Laravel/php, being:

• Set all myitems to empty (requiring the hidden input, so PHP receives an empty string for myitems)
• Update other properties of the model without touching myitems (so excluding any myitems input form the form. PHP will not receive the myitems key)

Sample code:

<input type="hidden" name="myitems" value="" />
<select name="myitems[]" multiple>
  <option value="1">Foo</option>
  <option value="2">Bar</option>
</select>

回答4:

You can add a - please select - entry and preselect it.

<select id="eng_0" name="eng_0[]" multiple size="3">
  <option value="nothing" selected="selected">- please select -</option>
  <option value="Privilégier">Privilégier</option>
  <option value="Accepté">Accepté</option>
  <option value="Temporaire">Temporaire</option>
</select>

回答5:

Add something like

    $("#field").change(function() {
      if (($("#field").val() || []) == "") $("form").append("<input type='hidden' name='field' value=''>");
    });

回答6:

what if even when the user selects it, nothing about the select arrives in the $_POST?

            <form action="cart.php" method="POST">
                <input type="hidden" id="acao" name="acao" value="add" />
                <input type="hidden" id="id" name="id" value="<?=$cnt->cnt_codigo?>" />
                <?php
                    $resOpc = mysql_query("SELECT * FROM loja_opcionais ORDER BY descricao")or die(mysql_error());
                    if(mysql_num_rows($resOpc) > 0){
                ?>
                <select id="opcional" nome="opcional">
                    <?php
                        while($opc = mysql_fetch_object($resOpc)){
                    ?>
                    <option value="<?=$opc->descricao?>"><?=$opc->descricao?></option>
                    <?php
                        }
                    ?>
                </select>
                <?php
                    }
                ?>
                <button class="botao" type="submit">Adicionar ao Carrinho</button>
            </form>

When I do print_r($_POST); on the other side the output is simply: Array ( [acao] => add [id] => 3 ) and no sign of the select tag, even when I change the value on the other side;

回答7:

if there is no $_POST, Post an empty string (nothing) is absolutely the most simple solution.

Here my solution for a Form with a <select name="related[]" multiple>

just add the following line in the php section that handles the storing of your form:

if (!isset($_POST['related'])) $_POST['related']="";

回答8:

Include <input type="hidden" name="yourfield" value="" /> to your form where the name is the same as for your multiple="multiple" select.

---

It is unfortunate that browsers do not send empty form parameters for multiple="multiple" selects like they do for non-multiple selects or a normal inputs.

Using a dummy value as proposed in the accepted answer is not a very clean solution.

(This question has nothing to do with jQuery)

来源：https://stackoverflow.com/questions/1132303/how-to-post-empty-select-multiple-html-elements-if-empty