PHP - Get remote IP address

get-ip.php

<?php
/**
 * Get remote IP address.
 *
 * @return string|null
 *
 * @link https://stackoverflow.com/a/41769505/12320578
 */
function getIp(): ?string {
  $ip_headers_keys = [
    'HTTP_CLIENT_IP',
    'HTTP_X_FORWARDED_FOR',
    'HTTP_X_FORWARDED',
    'HTTP_X_CLUSTER_CLIENT_IP',
    'HTTP_FORWARDED_FOR',
    'HTTP_FORWARDED',
    'REMOTE_ADDR',
  ];

  foreach ( $ip_headers_keys as $ip_header_key ) {
    if ( array_key_exists( $ip_header_key, $_SERVER ) !== true ) {
      continue;
    }

    foreach ( explode( ',', $_SERVER[ $ip_header_key ] ) as $ip ) {
      $ip = trim( $ip );

      if ( filter_var( $ip, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE ) === false ) {
        continue;
      }

      return $ip;
    }
  }

  return null;
}