/**
 * Updates user meta.
 *
 * @param int    $user_id User ID.
 * @param string $meta_key The key of the meta field.
 * @param mixed  $meta_value The value of the meta field.
 * @param mixed  $prev_value Optional. If specified, only update existing metadata entries with the specified value.
 * @return bool|int False on failure, or the meta ID on success.
 */
function update_user_meta( $user_id, $meta_key, $meta_value, $prev_value = '' );

$user_id = 1;
$meta_key = 'favorite_color';
$new_meta_value = 'red';
$prev_value = 'blue'; // 只有当值为'blue'时才更新

// 更新用户元信息
if ( update_user_meta( $user_id, $meta_key, $new_meta_value, $prev_value ) ) {
    echo "User meta updated successfully.";
} else {
    echo "No changes were made to the user meta.";
}

/**
 * Deletes user meta.
 *
 * @param int    $user_id User ID.
 * @param string $meta_key The key of the meta field.
 * @param mixed  $meta_value Optional. If specified, only delete the entry with the specified value.
 * @return bool True if deleted, false if not.
 */
function delete_user_meta( $user_id, $meta_key, $meta_value = '' );

$user_id = 1;
$meta_key = 'favorite_color';

// 删除用户元信息
if ( delete_user_meta( $user_id, $meta_key ) ) {
    echo "User meta deleted successfully.";
} else {
    echo "Failed to delete user meta.";
}