Authenticate callback for WordPress register_meta() function.

functions.php

<?php //* do not include php tag
 
/**
 * Callback for WordPress register_meta() sanitize parameter.
 *
 * Determine if the current meta key and value should be visible within the 
 * WordPress post editor Custom Fields meta box.
 * 
 * @see https://codex.wordpress.org/Function_Reference/register_meta
 *
 * @uses current_user_can()
 * @see https://codex.wordpress.org/Function_Reference/current_user_can
 * 
 * @param  boolean $allowed  True if allowed to view the meta field by default, false if else.
 * @param  string  $meta_key The meta key.
 * @param  integer $post_ID  The post ojbect ID.
 * @param  integer $user_id  The user ID.
 * @param  string  $cap      The meta capability.
 * @param  array   $caps     An array of capabilities.
 * @return boolean           True if allowed to view the meta field by default, false if else.
 */
function auth_sample_count_meta( $allowed, $meta_key, $post_ID, $user_id, $cap, $caps ) {
    
    if ( ! current_user_can( 'edit_others_posts' ) ) {
        $allowed = false;
    }
    
    return $allowed;
    
}