我正在尝试向类的属性添加apply\\u filters函数，以便可以从其他插件或函数向中添加表单字段。php，但我无法设置$filter\\u字段。有人能告诉我我做错了什么吗？当我回显$form\\u字段时，我得到h4和submit按钮，但没有字段。

班级登记表。php

class RegForm {


         // Properties
        private $fields;

        private $filter_fields;

         // Constructor
        function __construct(){

            $this->fields = array(
                \'name\'      => \'<label>Name.</label><br>
                                <input type="text" name="name"><br>\',
                \'business name\' => \'<label>Business Name.</label><br>
                                <input type="text" name="business_name"><br>\',
                \'email\'     => \'<label>Email.</label><br>
                                <input type="email" required="required" name="email"><br>\',
                \'password\'  => \'<label>Password.</label><br>
                                <input type="password" required="required" name="password"><br>\'
                );

            add_shortcode(\'registerform\', array($this, \'create_reg_form\'));

        }

          // Methods

            public function init (){
                $filter_fields = apply_filters(\'tbg_reg_fields\', $this->fields);
            }

            public function create_reg_form ($filter_fields){

                $form_fields = implode($this->filter_fields);


                ?>
                    <h4>Register as a New Store Owner</h4>
                        <form method="post" class="tbg_reg_form" action="\' . admin_url( \'admin-ajax.php\' ) . \'">
                            <?php echo $form_fields; ?>
                            <input type="hidden" name="action" value="tbg_form_submit">
                            <input type="submit" value="Start Selling Now">
                        </form>
                    <?php
                    }
                }

        $newForm = new RegForm;
        add_action (\'plugins_loaded\', [$newForm, \'init\']);

    // Registration Fields 

function add_age_field($new_fields) {
    $new_fields[] = \'<label>Website.</label><br><input type="text" name="website"><br>\';
    return $new_fields;
}

add_filter(\'tbg_reg_fields\', \'add_age_field\');