How to store multiple input values with same meta_key

Question

is there a way to store the input value from multiple custom meta box fields with the same meta_key? I use the following code to store ONE value for the meta_key 'startdate':

function startdate() {
  global $post;
  $custom = get_post_custom($post->ID);
  $startdate = $custom["startdate"][0];
  ?>

<label>Startdate</label><br/>
<input type="text" name="startdate" value="<?php echo $startdate; ?>"/>

<?php }

add_action('save_post', 'save_details');
function save_details(){
  global $post;
  update_post_meta($post->ID, "startdate", $_POST["startdate"]);
}

If i had a second input field, how can i store its value with a different meta_id but the same meta_key (startdate)? Thank you very much!

(If i use the built-in custom field functionality i can save multiple values for the same meta key...)

Answer 1

Change your form as suggested:

function startdate() {
    global $post;
    $custom = get_post_custom($post->ID);
    echo "<label>Startdates</label><br/>";
    for ($i=0; $i<count($custom["startdate"]);$i++) {
        echo "<input type=\"text\" name=\"startdate[".$i."]\" value=\"".$custom["startdate"][$i]."\" />";
    }
}

You'll have to remove and reinstate your individual postmeta entries:

add_action('save_post', 'save_details');

function save_details($post_id) {
    if ($parent_id = wp_is_post_revision($post_id)) $post_id = $parent_id;

    if (!empty($_POST['startdate']) && is_array($_POST['startdate'])) {
        delete_post_meta($post_id, 'startdate');
        foreach ($_POST['startdate'] as $startdate) {
            add_post_meta($post_id, 'startdate', $startdate);
        }
    }
}

Then, of course, you'll need to add some sort of add/remove mechanism to your metabox form, probably through JS.

Answer 2

As mentioned by t31os try changing your form inputs to use name="startdate[]" then loop through the array saving each value:

foreach ($_POST["startdate"] as $datevalue) {
    update_post_meta($post->ID, "startdate", $datevalue);
}