我已经创建了一个元框。代码为：

/**
 * Add meta box 
 *
 * @param post $post The post object
 * @link https://codex.wordpress.org/Plugin_API/Action_Reference/add_meta_boxes
 */
function portfolio_add_meta_boxes( $post ){
    add_meta_box( \'portfolio_meta_box\', __( \'Company URLs\', \'portfolio\' ), \'portfolio_build_meta_box\', \'portfolio\', \'side\', \'low\' );
}
add_action( \'add_meta_boxes_portfolio\', \'portfolio_add_meta_boxes\' );

/**
 * Build custom field meta box
 *
 * @param post $post The post object
 */
function portfolio_build_meta_box( $post ){
    // make sure the form request comes from WordPress
    wp_nonce_field( basename( __FILE__ ), \'portfolio_meta_box_nonce\' );

    // retrieve the  current value
    $fbportfolio = get_post_meta( $post->ID, \'fbportfolio\', true );
    $twportfolio = get_post_meta( $post->ID, \'twportfolio\', true );
    $instportfolio = get_post_meta( $post->ID, \'instaportfolio\', true );
    $linkinportfolio = get_post_meta( $post->ID, \'linkinportfolio\', true );

    ?>
    <div class=\'inside\'>

        <h3>Facebook</h3>
        <p>
        <input type="url" name="fbportfolio" value="<?php echo $fbportfolio; ?>">
        </p>

    
    
    </div>
    <?php
}

/**
 * Store custom field meta box data
 *
 * @param int $post_id The post ID.
 * @link https://codex.wordpress.org/Plugin_API/Action_Reference/save_post
 */
function portfolio_save_meta_box_data( $post_id ){

    // store custom fields values
    update_post_meta( get_the_ID(), \'fbportfolio\', $_POST[\'fbportfolio\'] );


}
add_action( \'save_post_food\', \'portfolio_save_meta_box_data\' );