Wordpress - How to save multiple select item value from metabox - wordpress

Question

I did a metabox that contains a multiple select item that takes his values from a custom post type.
It works but i'm unable to save it...can you help me?
function conduce_palinsesto($post){
?><p>Seleziona il/i conduttore/i</p>
<p>Tieni premuto CTRL per selezionare più conduttori</p>
<?php
global $post;
echo '<select name="conduce[]" id="conduce" multiple="yes">';
$val = get_post_meta($post->ID, 'speaker', true);
$q = get_posts('post_type=speaker');
foreach ($q as $obj)
{
echo '<option value="'.$obj->ID.'" "checked="checked">'.$obj->post_title.'</option>';
}
echo '</select>';
}
This is my save function...that don't work.
add_action('save_post', 'rb_speaker_save_details');
function rb_speaker_save_details($post_id)
{
if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
return $post_id;
} else {
$speak = implode(',', $_POST['conduce']);
update_post_meta($post_id, 'conduce', speak);
}
}

Answer 1

I know it's probably been long solved but here is the answer. I was having the same problem and this is how I solved it.
to save
function rb_speaker_save_details($post_id) {
$is_autosave = wp_is_post_autosave( $post_id );
$is_revision = wp_is_post_revision( $post_id );
$is_valid_nonce = ( isset( $_POST[ 'name_of_nonce' ] ) && wp_verify_nonce( $_POST[ 'name_of_nonce' ], basename( __FILE__ ) ) ) ? 'true' : 'false';
if ( $is_autosave || $is_revision || !$is_valid_nonce ) {
return;
}
if( isset($_POST['conduce']) ) {
$speak = implode(',', $_POST['conduce']);
update_post_meta($post_id, 'conduce', $speak);
}}
and to check the option based on the saved data
echo '<option value="', $slug, '" ';
if(strpos($conduce_val, $slug) !==false) { echo 'selected="selected"'; }
echo '>',the_title(), "</option>\n";

Answer 2

For saving data inserted/selected in a metabox you have to use the save_post action. Maybe you want to have a look at the metabox example in the WP codex.