foreach ($parks as $key => $val) {
    echo $key;
};

<?php
    $dbname = "DATABASE";
    $password = "PASSWORD";
    $servername = "localhost";
    $dbname = "NAME";

    $conn = new mysqli($servername, $username, $password, $dbname);

    $query = "SELECT name, latitude, longitude, description, where_to_go from locations";
    $result = mysqli_query($conn, $query);

    $parks = [];

    while($row = mysqli_fetch_array($result)){
        $park = new stdClass();
        $location = new stdClass();
        $location->lat = floatval($row['latitude']);
        $location->lng = floatval($row['longitude']);
        $park->title = $row['name'];
        $park->location = $location;
        $park->description = $row['description'];
        $park->whereToGo = $row['where_to_go'];
        array_push($parks, json_encode($park));
    };
    /*
    foreach ($parks as $key => $val) {
        echo $key; //THIS WILL WORK
    };

    foreach ($parks as $key => $val) {
        echo $val; //THIS WILL NOT RETURN ANYTHING
    };

    echo $parks[0] //SIMILARLY, THIS WILL NOT RETURN ANYTHING
    */

    $conn->close();
?>

<script type="text/javascript">
    var locations = <?php echo json_encode($parks); ?>;
    var locations = locations.map(JSON.parse);
    console.log(locations);
</script>