使用mysqli的SELECT返回null

Question

I'm trying to select a row from a table using mysqli but all I can get is a bunch of null values and I don't really know why. The same query works using normal php mysql and if I try to perform the same query on phpMyAdmin using the parameter I pass goes through fine.

Here's the code:

$con = mysqli_connect('localhost', 'user', 'pass',"db");

if (mysqli_connect_errno()){
    die("Failed to connect to MySQL: " . mysqli_connect_error());
}
$coupon = $_GET['coupon'];
$sql = mysqli_prepare($con, "SELECT * FROM coupon WHERE coupon=?");
$sql->bind_param('s', $coupon);
$sql->execute();
$sql->store_result();
echo $sql;

returns

"affected_rows":null,
"insert_id":null,
"num_rows":null,
"param_count":null,
"field_count":null,
"errno":null,
"error":null,
"error_list":null,
"sqlstate":null,
"id":null

I already tried to search for an answer either here and on google but I couldn't find anything close to my problem. What am I doing wrong?

Answer 1

author's final solution (moved from question content):

As suggested by user @RiggsFolly I wasn't fetching the results at all, plus I was mixing procedural and OOP approaches as suggested by user @Alex . Here's the working code for future reference to anyone who will arrive here with a similar problem:

$coupon = $_GET['coupon'];
if ($sql = mysqli_prepare($con, "SELECT * FROM coupon WHERE coupon=?;")){
    mysqli_stmt_bind_param($sql, 's', $coupon);
    mysqli_stmt_execute($sql);
    mysqli_stmt_bind_result($sql, $ID, $coupon, $discount, $uses);
    mysqli_stmt_fetch($sql);
    $data = array(
        'ID' => $ID,
        'coupon' => $coupon,
        'discount' => $discount,
        'uses' => $uses
    );
    echo json_encode($data);
}else{
    echo json_encode(FALSE);
}