A class for handling the addition of custom user registration fields in WordPress.

user-registration-field.php

<?php

/**
 * Class WP_Scholar_User_Registration_Field
 */
class WP_Scholar_User_Registration_Field {

	/**
	 * Field name
	 *
	 * @var string
	 */
	public $name;

	/**
	 * Render callback
	 *
	 * @var callable
	 */
	public $render;

	/**
	 * Sanitization callback
	 *
	 * @var callable
	 */
	public $sanitize = 'sanitize_text_field';

	/**
	 * Validation callback
	 *
	 * @var callable
	 */
	public $validate;

	/**
	 * Setup actions and filters
	 */
	public function __construct() {
		add_action( 'register_form', array( $this, 'render' ) );
		add_filter( 'registration_errors', array( $this, 'validate' ) );
		add_action( 'register_new_user', array( $this, 'save' ) );
	}

	/**
	 * Get the raw value from $_POST
	 *
	 * @return string
	 */
	function get_raw_value() {
		return trim( filter_input( INPUT_POST, $this->name, FILTER_SANITIZE_STRING ) );
	}

	/**
	 * Sanitize the raw registration field value
	 *
	 * @param string $raw
	 *
	 * @return mixed
	 */
	function sanitize( $raw ) {
		return is_callable( $this->sanitize ) ? call_user_func( $this->sanitize, $raw ) : null;
	}

	/**
	 * WordPress callback for rendering the registration field
	 */
	function render() {
		if ( is_callable( $this->render ) ) {
			call_user_func( $this->render, $this );
		}
	}

	/**
	 * WordPress callback for validating the registration field
	 *
	 * @param WP_Error $errors
	 *
	 * @return WP_Error
	 */
	function validate( WP_Error $errors ) {
		if ( is_callable( $this->validate ) ) {
			$errors = call_user_func( $this->validate, $errors );
		}

		return $errors;
	}

	/**
	 * WordPress callback for saving the registration field
	 *
	 * @param int $user_id
	 */
	function save( $user_id ) {
		$raw_value = $this->get_raw_value();
		$value = $this->sanitize( $raw_value );
		if ( ! empty( $value ) ) {
			update_user_meta( $user_id, $this->name, $value );
		}
	}

}