You need to incorporate the result from switch as the first parameter. I also changed $distance to $constant for clarity.
switch($unit) {//From User
case "km":
$constant = 6371; // KM
break;
case "mi":
$constant = 3959; // KM
break;
}
$query = "SELECT store_id, name, address, description, longitude, latitude,
(? * acos(cos(radians(?)) * cos( radians(latitude)) * cos(radians(longitude)
- radians(?)) + sin(radians(?)) * sin(radians(latitude))))
AS distanceFromLocation FROM stores";
$stmt = $mysqli->prepare($query);
$stmt->bind_param($constant, $distance, $user_latitude, $user_longitude, $user_latitude);
etc.