我试图创建自己的函数调用Test 从一个简单的插件php文件（列表类别插件），但当我调用它。显示错误” Fatal error: Uncaught Error: Call to undefined function… “

<?php
/* 
Plugin Name: List Categories
*/
class ListCategories{
   static function list_categories($atts, $content = null) {
    $atts = shortcode_atts(
      array(
        \'show_option_all\'    => \'\',
        \'orderby\'            => \'name\',
        \'order\'              => \'ASC\',
        \'style\'              => \'list\',
        \'show_count\'         => 0,
        \'hide_empty\'         => 1,
        \'use_desc_for_title\' => 1,
        \'child_of\'           => 0,
        \'feed\'               => \'\',
        \'feed_type\'          => \'\',
        \'feed_image\'         => \'\',
        \'exclude\'            => \'\',
        \'exclude_tree\'       => \'\',
        \'include\'            => \'\',
        \'hierarchical\'       => 1,
        \'title_li\'           => __( \'Categories\' ),
        \'show_option_none\'   => __( \'No categories\' ),
        \'number\'             => null,
        \'echo\'               => 1,
        \'depth\'              => 0,
        \'current_category\'   => 0,
        \'pad_counts\'         => 0,
        \'taxonomy\'           => \'category\',
        \'walker\'             => null
      ), $atts
    );

    test();
    ob_start();
    wp_list_categories($atts);
    $output = ob_get_contents();       
    ob_end_clean();    
    return $output; 
  }

  function test(){
    //Do something
  }
}

add_shortcode( \'categories\', array(\'ListCategories\', \'list_categories\') );