long poll hanging all other server requests

I have a simple long poll request on my website to check if there are any new notifications available for a user. As far as the request goes, everything seems to work flawlessly; the request is only fulfilled once the database is updated (and a notification has been created for that specific user), and a new request is sent out straight after.

The Problem

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What I have noticed is that when the request is waiting for a response from the database (as long polls should), all other requests to the server will also hang with it - whether it be media files, AJAX requests or even new pages loading. This means that all requests to the server will hang until I close my browser and reopen it.

What is even stranger is that if I visit another one of my localhost sites (my long poll is on a MAMP virtualhost site, www.example.com), their is no problem and I can still use them as if nothing has happened - despite the fact they're technically on the same server.

My Code

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This is what I have on my client side (longpoll.js):

window._Notification: {
    listen: function(){
        /* this is not jQuery's Ajax, if you're going to comment any suggestions, 
        *  please ensure you comment based on regular XMLHttpRequest's and avoid
        *  using any suggestions that use jQuery */
        xhr({
            url: "check_notifs.php",
            dataType: "json",
            success: function(res){
                /* this will log the correct response as soon as the server is 
                *  updated */
                console.log(res);
                _Notification.listen();
            }
        });
    }, init: function(){
        this.listen();
    }
}

/* after page load */
_Notification.init();

And this is what I have on my server side (check_notifs.php):

header("Content-type: application/json;charset=utf-8", false);

if(/* user is logged in */){
    $_CHECKED = $user->get("last_checked");

    /* update the last time they checked their notifications */
    $_TIMESTAMP = time();
    $user->update("last_checked", $_TIMESTAMP);

    /* give the server a temporary break */
    sleep(1);

    /* here I am endlessly looping until the conditions are met, sleeping every
    *  iteration to reduce server stress */
    while(true){
        $_PDO = new PDO('...', '...', '...');
        $query = $_PDO->prepare("SELECT COUNT(*) as total FROM table WHERE timestamp > :unix");
        if($query->execute([":unix" => $_CHECKED])){
            if($query->rowCount()){
                /* close the PDO connection */
                $_PDO = null;

                /* check if the database has updated and if it has, break out of
                *  the while loop */
                $total = $query->fetchAll(PDO::FETCH_OBJ)[0]->total;
                if($total > 0){
                    echo json_encode(["total" => $total]);
                    break;
                }
                /* if the database hasn't updated, sleep the script for one second,
                *  then check if it has updated again */
                sleep(1);
                continue;
            }
        }
    }
}   

/* for good measure */
exit;

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I have read about NodeJS and various other frameworks that are suggested for long-polling, but unfortunately they're currently out of reach for me and I'm forced to use PHP. I have also had a look around to see if anything in the Apache configuration could solve my problem, but I only came across How do you increase the max number of concurrent connections in Apache?, and what's mentioned doesn't seem like it would be the problem considering I can still use my other localhost website on the same server.

Really confused as to how I can solve this issue, so all help is appreciated,
Cheers.

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