add_post_meta One Key with Multiple Values

<?php
/*
Plugin Name: Your Plugin Name Here
Plugin URI: http://your-site.com
Description: Your description here.
Version: 1.0
Author: Your Name
Author URI: ttp://your-site.com
License: A "Slug" license name e.g. GPL2
*/

add_action('save_post','ez_portfolio_gallery_save');
add_action( 'add_meta_boxes', 'ez_portfolio_gallery_box' );

function ez_portfolio_gallery_save($post_ID){

    global $post;
    $ez_portfolio_gallery = $_POST['ez_portfolio_gallery'];

    //verify post is not a revision
    if ( !wp_is_post_revision( $post_ID ) ) {

        add_post_meta($post_ID, 'ez_portfolio_gallery', $ez_portfolio_gallery);

    }
}


function ez_portfolio_gallery_box(){
    add_meta_box( 'ez_portfolio_gallery_box', 'Portfolio Gallery', 'ez_portfolio_gallery_fields', 'post' );
}

function ez_portfolio_gallery_fields(){
?>

    <label for="ez_portfolio_gallery">Image URL:</label><br />
    <input type="text" class="widefat" id="ez_portfolio_gallery" name="ez_portfolio_gallery" value="" /><br />

<?php

    $mykey_values = get_post_custom_values( 'ez_portfolio_gallery', $post_ID );
    foreach ( $mykey_values as $key => $value ) {
    echo "$key  => $value <br />";
  }
}