Cách 1 : Lấy địa chỉ IP thông qua hàm getenv()

// Function to get the client IP address
function get_client_ip() {
	$ipaddress = '';
	if (getenv('HTTP_CLIENT_IP')) {
		$ipaddress = getenv('HTTP_CLIENT_IP');
	} else if (getenv('HTTP_X_FORWARDED_FOR')) {
		$ipaddress = getenv('HTTP_X_FORWARDED_FOR');
	} else if (getenv('HTTP_X_FORWARDED')) {
		$ipaddress = getenv('HTTP_X_FORWARDED');
	} else if (getenv('HTTP_FORWARDED_FOR')) {
		$ipaddress = getenv('HTTP_FORWARDED_FOR');
	} else if (getenv('HTTP_FORWARDED')) {
		$ipaddress = getenv('HTTP_FORWARDED');
	} else if (getenv('REMOTE_ADDR')) {
		$ipaddress = getenv('REMOTE_ADDR');
	} else {

		$ipaddress = 'UNKNOWN';
	}

	return $ipaddress;
}

Cách dùng

echo get_client_ip();

Cách 2 : Hướng dẫn lấy địa chỉ IP thông qua $_SERVER

// Function to get the client IP address
function get_client_ip() {
    $ipaddress = '';
    if (isset($_SERVER['HTTP_CLIENT_IP']))
        $ipaddress = $_SERVER['HTTP_CLIENT_IP'];
    else if(isset($_SERVER['HTTP_X_FORWARDED_FOR']))
        $ipaddress = $_SERVER['HTTP_X_FORWARDED_FOR'];
    else if(isset($_SERVER['HTTP_X_FORWARDED']))
        $ipaddress = $_SERVER['HTTP_X_FORWARDED'];
    else if(isset($_SERVER['HTTP_FORWARDED_FOR']))
        $ipaddress = $_SERVER['HTTP_FORWARDED_FOR'];
    else if(isset($_SERVER['HTTP_FORWARDED']))
        $ipaddress = $_SERVER['HTTP_FORWARDED'];
    else if(isset($_SERVER['REMOTE_ADDR']))
        $ipaddress = $_SERVER['REMOTE_ADDR'];
    else
        $ipaddress = 'UNKNOWN';
    return $ipaddress;
}