I think the flow would basically be:

• If the user already has a cookie referredyou skip the rest of the checks. $referredis the value of the cookie.
• If the user does not have that cookie but sent the parameter refin the query string, $referredit is the value of $_GET['ref'].
• If none of the above are true, it $referredis0000
• At the end of the process, you set the cookie in the response header with the value it $referredtook in the previous loop. This implies that if you visit weekly, your cookie is renewed for 30 days each time.

The code is more or less this:

if(isset($_COOKIE["referred"])) {
   $referred = $_COOKIE['referred'];
} else if (isset($_GET['ref'])) {
   $referred = $_GET['ref'];
} else {
   $referred = '0000';
}

setcookie('referred', $referred, time() + (86400 * 30), '/');

echo $referred;

Beware, if the user cleans the cookies of his browser, it is as if he had never visited your site.

edited bonus track

(fixes syntax errors and displays informational messages)

To handle this case you could use the frontend and if it detects the cookie, store the value in the localStoragebrowser. The localStorage forces the cookie to stubbornly persist . In that case the flow would be:

• I enter the site. I don't have cookies. The backend returns the cookie to me 0000.
• I check if I have the key referredin localStorage
• If I don't have it, I assign the value of the cookie that the backend gives me
• If I have it and if its expiration date has already expired, I flatten its value with what the backend gave me.
• If I have it and it hasn't expired , and its value is different from what the backend tells me , then I overwrite the cookie with what I have in localStorage. Subsequent visits or refreshes now detect the cookie from the backend.

However , you will not be able to directly read the expiration date of the browser cookie. What you can do then is give the cookie the value $referredconcatenated with the expiration date, preferably in the form of a timestamp. In this case, if the referrer is, 123you could assign the value 123_<timestamp de expiración>.

Full working example

(I changed the separator |to an underscore _because the pipe is rewritten to %7Cand that complicates everything)

In this case the complete script would be:

<?php
$action1 = 'fijó';
$action2 = 'fijó';
$referred = '';
// comprobamos si la cookie existe y tiene el formato valor_<expiración>
if (isset($_COOKIE['referred']) && strpos($_COOKIE['referred'], '_') !== false) {
    $cookie_array = explode('_', $_COOKIE['referred']);
    $referred = $cookie_array[0];
    $action1 = 'detectó';
    $action2 = 'extendió';
} else if (isset($_GET['ref'])) {
    $referred = $_GET['ref'];
    $action1 = 'detectó en el parámetro ref';
}
// si no hay referred o está vacío, fijamos el default
if (!$referred) {
    $action1 = 'fijó por defecto';
    $action2 = 'fijó';
    $referred = '0000';
}

$expiration = time() + (86400 * 30);
setcookie('referred', implode('_', [$referred, $expiration]), $expiration, '/');

echo '<p><b>Acción en el backend:</b> <span id="serverside">';
echo sprintf('se %s la cookie con valor %s y se %s su expiración a %s', $action1, $referred, $action2, $expiration);
echo '</span></p>';
?>
<p><b>Detección en el frontend:</b> <span id="cookie_deferred"></span></p>

<p><b>Detección en localStorage:</b> <span id="cookie_localstorage"></span></p>

<script>
    var mensaje, referred, timestamp = parseInt(Date.now()/1000,10), // js usa microsecundos
    cookieDeferredSelector=document.querySelector('#cookie_deferred'),
    localStorageSelector = document.querySelector('#cookie_localstorage'),
    referred_cookie = document.cookie.match(/referred([^;]+)/),
    stored_cookie = localStorage.getItem('referred');

// Si el browser tiene la cookie, sobreescribo el valor default
if(referred_cookie && referred_cookie.length) {
    referred = referred_cookie[1].replace(/^=/,'');
    mensaje='detectó la cookie "referred"';
} else {
    referred = '0000_'+(86400*30+timestamp); // default
    mensaje='no detectó la cookie "referred". Se fijó por defecto una';
}

var referred_array = referred.split('_');
    cookie_value = referred_array[0],
    correct_value = cookie_value,
    cookie_expiration= referred_array[1];

cookieDeferredSelector.innerText=`El browser ${mensaje} con valor ${cookie_value} y expiración ${cookie_expiration}`;
if(stored_cookie!==null) {
    stored_value = stored_cookie.split('_')[0],
    stored_expiration = stored_cookie.split('_')[1];
}


if (stored_cookie === null) {
    localStorageSelector.innerText=`localStorage no tiene una llave "deferred". Se asignó el valor de la cookie: ${referred}`;
// Si no hay nada en localStorage, le pongo el valor de la cookie
   localStorage.setItem('referred', referred);
} else if (stored_expiration > timestamp && stored_value.length && cookie_value!==stored_value) {
   correct_value = stored_value;
   var new_timestamp=timestamp+30*86400,
       new_expiration = new Date(new_timestamp*1000).toGMTString(),
       new_cookie = [stored_value, new_timestamp].join('_');

    localStorageSelector.innerText=`El valor en localStorage ${stored_value} es distinto al de la cookie ${cookie_value}. Se sobreescribió la cookie con ese valor`;
   // Si el localStorage no ha expirado y el backend me devolvió '0000'
   document.cookie=`referred=${new_cookie};expires=${new_expiration}`;
} else if (stored_expiration <= timestamp || !stored_value.length) {
    localStorageSelector.innerText=`El valor en localStorage expiró o no tiene valor. Se le asignó el valor de la cookie: ${referred}`;
    localStorage.setItem('referred', referred);

} else if (cookie_value===stored_value) {
    localStorageSelector.innerText=`El valor almacenado en localStorage coincide con el valor de la cookie: ${cookie_value}`;
    localStorage.setItem('referred', referred); // igual actualizamos la expiración en localStorage
}
</script>

And I think that completes the cycle. If you notice, I am declaring a variable correct_valuethat is the value of the cookie (only what comes before the underscore). If localStorage steps on the cookie, it also updates correct_value.

I pulled up that example at https://examples.ffflabs.com/setcookie.php

Some possible streams

Initial Flow you go to https://examples.ffflabs.com/setcookie.php?ref=1234

You enter for the first time. There is no value in $_COOKIES. It detects the ref 1234 and sets that value. The browser says:

Action in the backend: the cookie with value 1234 was detected in the ref parameter and its expiration was set to 1540046455

Frontend detection The browser detected the "referred" cookie with value 1234 and expiration 1540046455

Detection in localStorage localStorage does not have a "deferred" key. Cookie value was assigned: 1234_1540046455

If you open Chrome Devtools, Application tab Storage section, you will see the value of the cookie ( you may have to press the refresh button if the cookie does not appear)

And the value in localStorage:

Second Flow : you go to https://examples.ffflabs.com/setcookie.php?ref=4321 (you are changing the ref)

The backend already has the cookie 1234. Ignore the ref and the frontend says:

Backend Action : Cookie with value 1234 was detected and its expiration was extended to 1540046974

Frontend detection : The browser detected the "referred" cookie with value 1234 and expiration 1540046974

Detection in localStorage : The value stored in localStorage matches the cookie value: 1234

If you examine Chrome Devtools the value of the cookie and localStorage have not changed. You persisted the cookie even though another ref was specified in the URL.

Third Flow: You delete the cookie but keep the value in localStorage. You navigate to https://examples.ffflabs.com/setcookie.php?ref=4321

The front end says:

Action in the backend : the cookie with value 4321 was detected in the ref parameter and its expiration was set to 1540047450

Frontend detection : The browser detected the "referred" cookie with value 4321 and expiration 1540047450

Detection in localStorage : The value in localStorage 1234 is different from cookie 4321. The cookie was overwritten with that value

You reload the page and you will see that thanks to localStorage you restored the referrer 1234.

Fourth Flow: You modify the value of localStorage to2468_1540047515

Your cookie keeps saying 1234_1540047515. You navigate to https://examples.ffflabs.com/setcookie.php?ref=4321

The frontend answers you:

Backend action : Cookie with value 1234 was detected and its expiration was extended to 1540047771

Frontend detection : The browser detected the "referred" cookie with value 1234 and expiration 1540047771

Detection in localStorage : The value in localStorage 2468 is different from cookie 1234. The cookie was overwritten with that value

In the background, what's in localstorage (if it's current) rules over the cookie . If you reload, now the referrer is2468

Fifth Flow: set localstorage to a different value and an expired timestamp 1357_1500047942

The frontend responds:

Backend Action : Cookie with value 2468 was detected and its expiration was extended to 1540047942

Frontend detection : The browser detected the "referred" cookie with value 2468 and expiration 1540047942

Detection in localStorage : The value in localStorage has expired or has no value. Cookie value was assigned: 2468_1540047942

LocalStorage has been overwritten with the cookie value because it is expired and needs to be renewed:

Last Flow: you delete the cookie. You delete localStorage. You reload the page https://examples.ffflabs.com/setcookie.php?ref=1234

You get the same response as in the initial flow (although the timestamps have changed). If you check Chrome DevTools you will see that the cookie and localStorage stored what the backend just passed to them.

Closing Conclusions

When the entire stream ends, the variable correct_valuealways has the correct value of the cookie.

• The initial value if it is the first visit
• The original value if the visitor cleared the cookie or the server was restarted or the server session timed out
• The new value if 30 days have passed since the last visit

So, if your original intention was to associate a user to a referrer in a semi-permanent way, then at the end of the flow you can send the data correct_valueto another endpoint (eg asociar_referrer.php) through Ajax that persists the user and his referrer, for example in a database. .

This persistence should allow insert if there is no value for that user, or update if it already exists . In MySQL that can be done using something like

INSERT INTO usuario_ref (id_usuario, referrer) VALUES (1, '1234')
  ON DUPLICATE KEY UPDATE referrer='1234';

If the userid is a primary key, it will insert the record if it doesn't exist, and replace it if it already exists.

The same in PostgreSQL has a similar syntax:

INSERT INTO usuario_ref (id_usuario, referrer) VALUES (1, '1234')
  ON CONFLICT (id_usuario) DO UPDATE SET referrer='1234';