extra-user-reg.php

<?php

/**
 * Plugin Name: Extra User Registration Source Tracking
 * Description: Store data about a user's origin -- IP Address and User Agent -- when registering.
 * Author: georgestephanis
 * Author URI: http://wpspecialprojects.wordpress.com/
 * License: GPLv2+
 */

add_action( 'user_register', 'to51_track_source_user_register' );
function to51_track_source_user_register( $user_id ) {
	$source_data = array(
		'REMOTE_ADDR' => $_SERVER['REMOTE_ADDR'],
		'USER_AGENT'  => $_SERVER['HTTP_USER_AGENT'],
	);

	// In order of preference, with the best ones for this purpose first.
	$address_headers = array(
		'HTTP_CLIENT_IP',
		'HTTP_X_FORWARDED_FOR',
		'HTTP_X_FORWARDED',
		'HTTP_X_CLUSTER_CLIENT_IP',
		'HTTP_FORWARDED_FOR',
		'HTTP_FORWARDED',
	);

	foreach ( $address_headers as $header ) {
		if ( array_key_exists( $header, $_SERVER ) ) {
			$source_data[ $header ] = $_SERVER[ $header ];
		}
	}

	add_user_meta( $user_id, '_registration_source_data', $source_data, true );

	wp_mail(
		get_bloginfo( 'admin_email' ),
		sprintf(
			'Registration from %1$s on "%2$s" for UID %3$d',
			$source_data['REMOTE_ADDR'],
			get_bloginfo( 'name' ),
			$user_id
		),
		wp_json_encode( $source_data, JSON_PRETTY_PRINT )
	);

}