Best way to Identify a user uniquely

[Insibisteboom]

I am building a Like/Dislike feature. For this purpose I need to identify the particular user liking or disliking the post. I have an idea that without user logins this cannot be done perfectly, but whats the closest to perfect technique, Issues I have are stated below:

• Cookies can be cleared, and vary form browser to browser
• Most of the users dont have a dedicated public IP, and their IP keeps on changing for every session
• a combo of cookies and IP might also not serve the purpose

So which technique shall I use to implement it ?The language I am using for this is PHP