What is the most accurate way to get a user IP address in PHP?

Simple way to get the user’s IP address:

function get_ip_address() {
  foreach (['HTTP_CLIENT_IP', 'HTTP_X_FORWARDED_FOR', 'HTTP_X_FORWARDED', 'HTTP_X_CLUSTER_CLIENT_IP', 'HTTP_FORWARDED_FOR', 'HTTP_FORWARDED', 'REMOTE_ADDR'] as $key) {
    if (array_key_exists($key, $_SERVER)) {
      foreach (explode(',', $_SERVER[$key]) as $ip) {
        $ip = trim($ip); // just to be safe
        if (filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE) !== false) {
          return $ip;
        }
      }
    }
  }
}