Extra registration fields

extra-registration-fields.php

<?php

add_action( 'register_form', 'additional_profile_fields' );
add_action( 'signup_extra_fields', 'additional_profile_fields' );
function additional_profile_fields() { ?>
  <p>
		<label><?php _e('First Name') ?><br />
		<input type="text" name="first_name" id="first_name" class="input" size="25" /></label>
	</p>
	<p>
		<label><?php _e('Last Name') ?><br />
		<input type="text" name="last_name" id="last_name" class="input" size="25" /></label>
	</p>
<?php }

add_action( 'register_post', 'add_register_field_validate_first_name', 10, 3 );
function add_register_field_validate_first_name( $sanitized_user_login, $user_email, $errors) {
	if (!isset($_POST[ 'first_name' ]) || empty($_POST[ 'first_name' ])) {
		return $errors->add( 'firstnameempty', '<strong>ERROR</strong>: Please provide a first name.' );
	}
}
add_action( 'register_post', 'add_register_field_validate_last_name', 10, 3 );
function add_register_field_validate_last_name( $sanitized_user_login, $user_email, $errors) {
	if (!isset($_POST[ 'last_name' ]) || empty($_POST[ 'last_name' ])) {
		return $errors->add( 'lastnameempty', '<strong>ERROR</strong>: Please provide a last name.' );
	}
}

add_filter( 'add_signup_meta', '_add_signup_meta' );
function _add_signup_meta() {
	$meta['last_name'] = $_POST['last_name'];
	$meta['first_name'] = $_POST['first_name'];
	return $meta;
}

add_action( 'user_register', 'insert_register_fields' );
function insert_register_fields( $user_id ) {
	if ( isset( $_POST['last_name'] ) )
		_insert_meta_on_activation( $user_id, $_POST );
}

add_action( 'wpmu_activate_user', 'insert_meta_on_activation', 10, 3 );
function insert_meta_on_activation( $user_id, $password, $meta ) {
	_insert_meta_on_activation( $user_id, $meta );
}

function _insert_meta_on_activation( $user_id, $meta ) {
	$first_name = apply_filters('pre_user_first_name', $meta['first_name']);
	$last_name = apply_filters('pre_user_last_name', $meta['last_name']);

	update_user_meta( $user_id, 'first_name', $first_name );
	update_user_meta( $user_id, 'last_name', $last_name );
}