Multi select meta box - wordpress

Question

I have added multi select meta box to custom post-type it appears good but save only one selection . How to change the code below to save all selected options.
add_action( 'add_meta_boxes', 'add_condition_metaboxes'); function add_scondition_metaboxes() {
add_meta_box('condition', 'Информация спикеров', 'wpt_condition_author', 'condition', 'side', 'default');} function wpt_scondition_author() {
global $post;
// Noncename needed to verify where the data originated
echo '<input type="hidden" name="conditionmeta_noncename" id="conditionmeta_noncename" value="' .
wp_create_nonce( plugin_basename(__FILE__) ) . '" />';
// Get the author data if its already been entered
$my_dropdown = get_post_meta($post->ID, '_dappcf_i_dropdown', true);
// Echo out the field
$posts = get_posts(array('post_type'=> 'speakers', 'post_status'=> 'publish', 'suppress_filters' => false, 'posts_per_page'=>-1));
//here you add the HTML of the dropdown you add something like
echo '<p>Select the speaker: <select multiple="yes" name="_dappcf_i_dropdown" class="widefat" style="width:170px">';
foreach ($posts as $post) {
echo '<option value="', $post->ID, '">'.$post->post_title.'</option>'; }
echo '</select>'; } function wpt_save_condition_meta($post_id, $post) {
// verify this came from the our screen and with proper authorization,
// because save_post can be triggered at other times
if ( !wp_verify_nonce( $_POST['conditionmeta_noncename'], plugin_basename(__FILE__) )) {
return $post->ID;
}
// Is the user allowed to edit the post or page?
if ( !current_user_can( 'edit_post', $post->ID ))
return $post->ID;
// OK, we're authenticated: we need to find and save the data
// We'll put it into an array to make it easier to loop though.
$condition_meta['_dappcf_i_dropdown'] = $_POST['_dappcf_i_dropdown'];
// Add values of $testimonial_meta as custom fields
foreach ($condition_meta as $key => $value) { // Cycle through the $testimonial_meta array!
if( $post->post_type == 'revision' ) return; // Don't store custom data twice
$value = implode(',', (array)$value); // If $value is an array, make it a CSV (unlikely)
if(get_post_meta($post->ID, $key, FALSE)) { // If the custom field already has a value
update_post_meta($post->ID, $key, $value);
} else { // If the custom field doesn't have a value
add_post_meta($post->ID, $key, $value);
}
if(!$value) delete_post_meta($post->ID, $key); // Delete if blank
} } add_action('save_post', 'wpt_save_condition_meta', 1, 2); // save the custom fields
The options are queried from other custom post-type and the number of post can be changed.