Context

I have a debate system on my website where users can click on the option of the debate they want to vote for, which then reveals the tally of the debate votes on either option. However, users need to log into their account for this to work, so users who are just passing by are unable to vote (you can check the functionality here). I'm looking to alter this code so that users can vote on debates without logging into their account. Knowing that most options are not completely reliable and can be worked around, I decided to settle on using cookies to store information that would identify if the users have voted on the debate or not without being logged in.

Question

As I am new to working with cookies, I was wondering: What is the best way to implement cookies so that I can insert the debate votes into my database and check against it when the non-logged-in user clicks on the same debate twice? I figured that I would just need a unique number that would be stored as a cookie on the client side until the cookies are removed. With this in mind, I thought I could just create a random number based on the user IP address, like so:

setcookie('temp_id', md5($_SERVER['REMOTE_ADDR']);

I know that users can change the IP address, but I didn't think this would matter as long as there's a number stored on the client side. Is this a good way to obtain a unique number that can be stored as a cookie on the client side, or should I be setting a different unique number that would work better for some reason? Is this a safe thing to do? There doesn't seem to be any important private user information in there.

The next part of the problem is how I would alter my database code to accomodate those who are not logged in. My database table poll_votes is currently configured like so:

CREATE TABLE poll_votes (
    vote_id INT(11) AUTO_INCREMENT PRIMARY KEY NOT NULL,
    yes VARCHAR(50) NOT NULL,
    no VARCHAR(50) NOT NULL,
    poll_id INT(11) NOT NULL,
    idUsers INT(11) NOT NULL,
    article_id INT(11) UNSIGNED,
    seen TINYTEXT NOT NULL,
    FOREIGN KEY (article_id) REFERENCES articles (article_id) ON DELETE CASCADE ON UPDATE CASCADE,
    FOREIGN KEY (poll_id) REFERENCES polls (poll_id) ON DELETE CASCADE ON UPDATE CASCADE,
    FOREIGN KEY (idUsers) REFERENCES users (idUsers) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB;

As you can see, one of the colums in my table is idUsers, which references the registered users in the table users. This means that I will have to alter the table by adding another column that would keep track of the unique cookie ID when the user votes on a debate and also set idUsers to NULL, correct? I wanted to get a check on this before I implemented it to be sure I wasn't misunderstanding how cookies could be used in this situation to solve this problem. Let me know if there's any confusion. Thanks.