我编写了以下js代码

jQuery(".selectbox").change(function(){
 var id = this.id;
var key_id=id;
 var selectname=\'\';

     jQuery.post(
    // see tip #1 for how we declare global javascript variables
    MyAjax.ajaxurl,
    {
        // here we declare the parameters to send along with the request
        // this means the following action hooks will be fired:
        // wp_ajax_nopriv_myajax-submit and wp_ajax_myajax-submit
        action : \'get-mata-value\',

        // other parameters can be added along with "action"
        prev_metakey : jQuery(this).val(),
        metakey: key_id
    },
    function( result ) {
       if(result[\'success\'] != false)
        {
            jQuery.each(result,function(index,value){
                jQuery(\'#\' +id).append(\'<option value="\'+value+\'">\'+value+\'</option>\');
            });
        }
    }
);

其功能是：

wp_localize_script( \'script\', \'MyAjax\',
    array( 
        \'ajaxurl\' => admin_url( \'admin-ajax.php\' ),
    )
);
wp_enqueue_script( \'script\', get_template_directory_uri() . \'/js/vehicle_parts.js\', array ( \'jquery\' ), 1.1, true);


    add_action("wp_ajax_get-mata-value", "get_mata_value");
    add_action("wp_ajax_nopriv_get-mata-value", "get_mata_value");
    function get_mata_value()
    {   
        global $wpdb;
    $key=$_POST["metakey"];
        $prev_value=$_POST["prev_metakey"];
        $result=array();

     $result=$wpdb->get_col("SELECT DISTINCT meta_value FROM $wpdb->postmeta WHERE meta_key = \'$key\'");

     return($result);

    }