<td>
    <select id="major" onChange="updateCat();">
        <?php foreach ($dropdown_major->fetchAll() as $drop_major): ?>
                <option
                    value=""
                    data-name="<?php echo $drop_major ['Major Category'];?>"
                >
                    <?php echo $drop_major ['Major Category'];?>
                </option>
        <?php endforeach; ?>
    </select>
</td>


<td>
    <select id="minor">
        <?php foreach ($dropdown_minor->fetchAll() as $drop_minor): ?>
                <option
                    value=""
                    data-name="<?php echo $drop_minor ['Minor Category'];?>"
                >
                    <?php echo $drop_minor ['Minor Category'];?>
                </option>
        <?php endforeach; ?>
    </select>
</td>

function updateCat() {
    var e = document.getElementById("major");
    var majorSelected = e.options[e.selectedIndex];

    document.getElementById("minor").value = majorSelected.dataset.name;
}

<?php
$host="xxxxxxxxxxx";
$dbName="xxxxx";
$dbUser="xxxxxxxxxxxxx";
$dbPass="xxxxxxxx";


$dbh = new PDO( "sqlsrv:server=".$host."; Database=".$dbName, $dbUser, $dbPass);
$dbh->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );
$sql_major = "SELECT DISTINCT [Major Category] FROM ProductTable ORDER BY [Major Category] ASC";
$sql_minor = "SELECT DISTINCT [Minor Category] FROM ProductTable ORDER BY [Minor Category] ASC";

$dropdown_major = $dbh->query($sql_major);
$dropdown_minor = $dbh->query($sql_minor);
?>