Callback function for 'meta_box_cb' argument inside register_taxonomy() that replaces the regular checkboxes with a plain dropdown list

wp-taxonomy-dropdown.php

<?php
/**
 * Callback function for taxonomy meta boxes
 * 
 * A simple callback function for 'meta_box_cb' argument
 * inside register_taxonomy() that replaces the regular
 * checkboxes with a plain dropdown list
 * 
 * @param  [type] $post [description]
 * @param  [type] $box  [description]
 * @link   http://wordpress.stackexchange.com/a/148965
 */
function my_taxonomy_meta_box_callback( $post, $box ) {

	$defaults = array( 'taxonomy' => 'category' );
	
	if ( ! isset( $box['args'] ) || ! is_array( $box['args'] ) ) {
	    $args = array();
	}
	else {
	    $args = $box['args'];
	}

	extract( wp_parse_args($args, $defaults), EXTR_SKIP );
	$tax = get_taxonomy( $taxonomy );
	?>
	
	<div id="taxonomy-<?php echo $taxonomy; ?>" class="categorydiv">
	        <?php 
	        $name = ( $taxonomy == 'category' ) ? 'post_category' : 'tax_input[' . $taxonomy . ']';
	        echo "<input type='hidden' name='{$name}[]' value='0' />"; // Allows for an empty term set to be sent. 0 is an invalid Term ID and will be ignored by empty() checks.
	        
	        $term_obj = wp_get_object_terms( $post->ID, $taxonomy ); //_log($term_obj[0]->term_id)
	
	        if ( ! empty( $term_obj ) ) {
		        wp_dropdown_categories( array(
		        	'taxonomy'		=> $taxonomy,
		        	'hide_empty'		=> 0,
		        	'name'			=> "{$name}[]",
		        	'selected'		=> $term_obj[0]->term_id,
		        	'orderby'		=> 'name',
		        	'hierarchical'		=> 0,
		        	'show_option_none'	=> '&mdash;',
		        	'class'			=> 'widefat'
		        ) );
	        }
	        ?>
	</div>
	<?php
}
?>

Do you know by any chance the default code? I want to edit a bit like add some echos here and there but not change it to a dropdown like the code above does.

@bustapaladin they're in the codex:

• Post cats meta box
• Post tags meta box

why does it add another category automatically?

when I publish the post it automatically creates another taxonomy in the list.

Hey eduardo..this is really great!!

I was wondering, is there a way to retain the add_new_item link that you normally get when you use register_taxonomy?

i.e.

'labels' => array(
'name' => 'Category Sorting',
'new_item_name' => 'New Category',
'add_new_item' => 'Add New Category',
),

Thanks so much for your help.