您的上述代码应按原样工作。

但是，在保存函数的末尾有一个逻辑错误-您使两个字段的保存取决于$_POST[\'_url\'] 已设置
我想这不是故意的。您可能需要以下内容：

if ( isset( $_POST[\'_url\'] ) ) {
    update_post_meta( $post_id, \'_url\', $_POST[\'_url\'] );
} else {
    delete_post_meta( $post_id, \'_url\' );
}

if ( isset($_POST[\'_testimonial\'] ) ) {
    update_post_meta( $post_id, \'_testimonial\', $_POST[\'_testimonial\'] );
} else {
    delete_post_meta( $post_id, \'_testimonial\' );
}

class WPSE_95952_meta
{
    private $fields = array(
        array(
            \'name\' => \'_url\',
            \'label\' => \'URL\'
        ),
        array(
            \'name\' => \'_testimonial\',
            \'label\' => \'Testimonial\'
        )
    );

    public function __construct()
    {
        // add a meta box for WordPress \'project\' type
        add_action( \'add_meta_boxes\', array( $this, \'add_project_box\' ) );
        // add a callback function to save any data a user enters in
        add_action( \'save_post\', array( $this, \'portfolio_meta_save\' ) );
    }

    public function add_project_box()
    {
        add_meta_box(
            \'portfolio_meta\',
            \'Project Infos\',
            array( $this, \'portfolio_meta_setup\' ),
            \'project\',
            \'side\',
            \'low\'
        );
    }

    public function portfolio_meta_setup()
    {
        global $post;

        $output = \'<div class="portfolio_meta_control">\';

        foreach ( $this->fields as $field ) {
            $output .= \'<label>\' . $field[\'label\'] . \'</label>\' .
                \'<p>\' .
                    \'<input type="text" name="\' . $field[\'name\'] .
                        \'" value="\' . get_post_meta( $post->ID, $field[\'name\'], true ) .
                        \'" style="width: 100%;" />\' .
                \'</p>\';
        }

        $output .= \'</div>\';

        // create for validation
        $output .= \'<input type="hidden" name="meta_noncename" value="\' .
            wp_create_nonce(__FILE__) . \'" />\';

        echo $output;
    }

    public function portfolio_meta_save( $post_id )
    {
        // exit on autosave
        if ( defined(\'DOING_AUTOSAVE\') && DOING_AUTOSAVE ) {
            return $post_id;
        }

        // check nonce
        if (
            ! isset( $_POST[\'meta_noncename\'] ) ||
            ! wp_verify_nonce( $_POST[\'meta_noncename\'], __FILE__ )
        ) {
            return $post_id;
        }

        // check capabilities
        if (
            ( \'post\' === $_POST[\'post_type\'] && ! current_user_can( \'edit_post\', $post_id ) ) ||
            ! current_user_can( \'edit_page\', $post_id ) 
        ) {
            return $post_id;
        }

        foreach ( $this->fields as $field ) {
            $old = get_post_meta( $post_id, $field[\'name\'], true );
            $new = isset( $_POST[$field[\'name\']] ) ? $_POST[$field[\'name\']] : false;

            if ( $new && $new != $old ) {
                update_post_meta( $post_id, $field[\'name\'], $new );
            } elseif ( ! $new && $old ) {
                delete_post_meta( $post_id, $field[\'name\'], $old );
            }
        }
    }
}

$wpse_95952_meta = new WPSE_95952_meta();