Görsel düzene pek takılmayın onu seyrim için yaptım.

<?php
$db = new PDO('mysql:host=localhost;dbname=gazete', 'root', '');
$db->query('SET NAMES utf8');


if (isset($_REQUEST['gazeteler'])) {
    header('Content-type: application/json');

    $rows = array();
    foreach($db->query('SELECT * FROM gazete ORDER BY gazete_ad ASC', PDO::FETCH_ASSOC) as $row) {
        array_push($rows, $row);
    }

    exit(json_encode($rows));
} elseif (isset($_REQUEST['gazete_id'])) {
    header('Content-type: application/json');

    $rows = array();
    foreach($db->query('SELECT * FROM gazeteci WHERE gazete_id = ' . (int) $_GET['gazete_id'] . ' ORDER BY gazeteci_ad ASC', PDO::FETCH_ASSOC) as $row) {
        array_push($rows, $row);
    }

    exit(json_encode($rows));
}

/* MySQLi
$db = new mysqli('localhost', 'root', '', 'gazete');
$db->query('SET NAMES utf8');

if (isset($_REQUEST['gazeteler'])) {
    header('Content-type: application/json');

    $query = $db->query('SELECT * FROM gazete ORDER BY gazete_ad ASC');

    $rows = array();
    while ($row = $query->fetch_object()) {
        array_push($rows, $row);
    }

    exit(json_encode($rows));
} elseif (isset($_REQUEST['gazete_id'])) {
    header('Content-type: application/json');

    $query = $db->query('SELECT * FROM gazeteci WHERE gazeteci_gazeteid = ' . (int) $_GET['gazete_id']);

    $rows = array();
    while ($row = $query->fetch_object()) {
        array_push($rows, $row);
    }

    exit(json_encode($rows));
}*/
?>
<!DOCTYPE html>
<html>
<head>
    <title>Gazeteci</title>
    <link rel="stylesheet" type="text/css" href="https://fonts.googleapis.com/css?family=Ubuntu:400,500&subset=latin-ext">
    <link rel="stylesheet" type="text/css" href="https://unpkg.com/nprogress@0.2.0/nprogress.css">
    <meta charset="utf-8">
    <style>
        body{font-family:Ubuntu;font-size:13px;background-color:#f0f0f0}
        form{max-width:340px;padding:35px;box-sizing:border-box;margin:0 auto;margin-top:10%;background-color:#fff;border-radius:4px;box-shadow:0 2px 4px rgba(0,0,0,0.08),0 0 2px rgba(0,0,0,0.08)}
        form .input{position:relative;margin-bottom:1em}
        form .input.hidden:after{position:absolute;content:"";width:100%;height:100%;top:0;left:0;background:#fff;opacity:.5;cursor:not-allowed}
        form .input label{display:block;margin-bottom:5px;margin-left:4px;color:#747474;font-weight:400}
        form .input select{-webkit-appearance:none;cursor:pointer;background-image:url(data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHZpZXdCb3g9IjAgMCAyMi44NzEgMTIuNDUzIj48cGF0aCBkYXRhLW5hbWU9IlBhdGggMTQ4OSIgZD0iTTExLjk1NyAxMi4yMkwyMi42NTEgMS4zMDlhLjc3Ljc3IDAgMCAwLTEuMS0xLjA3N0wxMS40MDUgMTAuNTgzIDEuMzE5LjI5M2EuNzcuNzcgMCAwIDAtMS4xIDEuMDc3bDEwLjYzNiAxMC44NTFhLjc2Ny43NjcgMCAwIDAgLjU1LjIzMS43NzcuNzc3IDAgMCAwIC41NTItLjIzMnoiIGZpbGw9IiM0MzQzNDMiLz48L3N2Zz4=);background-repeat:no-repeat;background-position:right 10px center;background-size:10px;width:100%;border-radius:4px;padding:0 10px;height:35px;line-height:35px;font-size:inherit;font-family:inherit;color:#495057;background-color:#fff;box-sizing:border-box;border:1px solid #ddd}
        form .input select:focus{outline:none}
        form button{box-sizing:border-box;font-family:inherit;font-size:13px;border-radius:4px;padding:10px 15px;border:0;color:#fff;font-weight:500;cursor:pointer;width:100%;background-color:#000;opacity:.2;cursor:not-allowed}
        form button.active{opacity:1;cursor:pointer;background-color:#1ABC9C}
    </style>
</head>
<body>
    <form>
        <div class="input">
            <label>Gazete</label>
            <select name="gazete_id">
                <option>Seçiniz</option>
            </select>
        </div>
        <div class="input hidden">
            <label>Gazeteci</label>
            <select name="gazeteci_id">
                <option>Seçiniz</option>
            </select>
        </div>
        <button type="button">Uygula</button>
    </form>

    <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js"></script>
    <script src="https://unpkg.com/nprogress@0.2.0/nprogress.js"></script>
    <script>
        NProgress.configure({
            parent: 'form'
        });

        $(function() {
            NProgress.start();
            $.getJSON("?gazeteler", function(json) {
                $.each(json, function(index, row) {
                    $("select[name=gazete_id]").append($("<option>").val(row.gazete_id).text(row.gazete_ad));
                });

                NProgress.done();
            });

            $("select[name=gazete_id]").change(function() {
                NProgress.start();
                $.getJSON("", {gazete_id: $(this).val()}, function(json) {
                    $("select[name=gazeteci_id]").empty().append($("<option>").text("Seçiniz (" + json.length + ")"));
                    $.each(json, function(index, row) {
                        $("select[name=gazeteci_id]").append($("<option>").val(row.gazeteci_id).text(row.gazeteci_ad));
                    });

                    $(".input.hidden").removeClass("hidden");

                    NProgress.done();
                });
            });

            $("select[name=gazeteci_id]").change(function() {
                $("button").toggleClass("active");
            });

            $("form button").click(function(event) {
                event.preventDefault();

                if (!$(this).hasClass("active")) return;

                console.log($("form").serialize());
            });
        });
    </script>
</body>
</html>