wp_dropdown_category multiple

exemple.php

<?php

wp_dropdown_categories( array(
	'taxonomy'   => 'category',
	'multiple'   => true,
	'walker'     => new Willy_Walker_CategoryDropdown(),
	'selected'   => array( 10, 12 ), // selected terms…
	'hide_empty' => false,
) );

/**
 * pour un meilleur style, vous pouvez utiliser select2 
 * … (et ajouter un attribut 'class' => 'select2-multiple' dans les arguments)
 */

multiple-wp-dropdown-categories.php

<?php

// This filter allow a wp_dropdown_categories select to return multiple items
add_filter( 'wp_dropdown_cats', 'willy_wp_dropdown_cats_multiple', 10, 2 );
function willy_wp_dropdown_cats_multiple( $output, $r ) {
	if ( ! empty( $r['multiple'] ) ) {
		$output = preg_replace( '/<select(.*?)>/i', '<select$1 multiple="multiple">', $output );
		$output = preg_replace( '/name=([\'"]{1})(.*?)\1/i', 'name=$2[]', $output );
	}
	return $output;
}

// This Walker is needed to match more than one selected value
class Willy_Walker_CategoryDropdown extends Walker_CategoryDropdown {
	public function start_el( &$output, $category, $depth = 0, $args = array(), $id = 0 ) {
		$pad = str_repeat('&nbsp;', $depth * 3);

		/** This filter is documented in wp-includes/category-template.php */
		$cat_name = apply_filters( 'list_cats', $category->name, $category );

		if ( isset( $args['value_field'] ) && isset( $category->{$args['value_field']} ) ) {
			$value_field = $args['value_field'];
		} else {
			$value_field = 'term_id';
		}

		$output .= "\t<option class=\"level-$depth\" value=\"" . esc_attr( $category->{$value_field} ) . "\"";

		// Type-juggling causes false matches, so we force everything to a string.
		if ( in_array( $category->{$value_field}, (array)$args['selected'], true ) )
			$output .= ' selected="selected"';
		$output .= '>';
		$output .= $pad.$cat_name;
		if ( $args['show_count'] )
			$output .= '&nbsp;&nbsp;('. number_format_i18n( $category->count ) .')';
		$output .= "</option>\n";
	}
}

This was very helpful to get a select2 dropdown menu working. Thank you!