Wordpress custom fields.

dummy.php

<?php
/**
 * Added custom metaboxes to post types on admin init action.
 */
function dummy_add_metabox() {
    add_meta_box("metabox-id", "Metabox name", "dummy_metabox_function", "page");
}

add_action("admin_init", "dummy_add_metabox");

function dummy_custom_fields() {
    return array( 
        'field_id1' => __('Field 1','dummy'),
        'field_id2' => __('Field 2','dummy'),
        'field_id3' => __('Field 3','dummy'),
        'field_id4' => __('Field 4','dummy'),
    );
}
 
/**
 * Custom metabox callback function.
 */
function dummy_metabox_function() {
    global $post;
    $fields = dummy_custom_fields(); ?>
    <table>
        <tbody><?php
        foreach( $fields as $field_id => $label ) : ?>
            <tr>
                <th style='text-align:right;' scope='row'><label for='<?php echo $field_id; ?>'><?php echo $label ?></label></th>
                <td><input type='text' id='<?php echo $field_id; ?>' name='<?php echo $field_id; ?>' value='<?php echo esc_attr($post->$field_id); ?>'></td>
            </tr>
        <?php endforeach; ?>
        </tbody>
    </table><?php
}

/**
 * Save post action callback.
 */
function dummy_save_post( $post_id, $post ) {
    if (!defined('DOING_AUTOSAVE') && !DOING_AUTOSAVE)
        return $postID;
    if ( !current_user_can( 'edit_page', $post_id ) )
        return;
    
    $fields = dummy_custom_fields();
    foreach ( array_keys($fields) as $field_id ) {
        if (isset($_POST[$field_id])) {
            if (!empty($_POST[$field_id])) {
                if ($_POST[$field_id] != $post->$field_id) { 
                    update_post_meta($post_id, $field_id, $_POST[$field_id]);
                }
            }
            else {
                delete_post_meta($post_id, $field_id );
            }
        }
    }
}

add_action('save_post', 'dummy_save_post', 10, 2);
 
/**
 * To show custom field in template use this code:
 *  <?php 
 *  $fields = dummy_custom_fields(); 
 *  foreach ( $fields as $field_id => $label ) : 
 *      if (isset($post->$field_id)) : ?>
 *          <p><?php echo $post->$field_id; ?></p>
 *      <?php endif;
 *  endforeach; ?>
 */