How can I with mysqli make a query with LIKE and get all results?

Here’s how you properly fetch the result

$param = "%{$_POST['user']}%";
$stmt = $db->prepare("SELECT id, username FROM users WHERE username LIKE ?");
$stmt->bind_param("s", $param);
$stmt->execute();

$result = $stmt->get_result();
while ($row = $result->fetch_assoc()) {
    echo "Id: {$row['id']}, Username: {$row['username']}";
}

or, if you prefer the old fetch and bind_result syntax, you can also do:

$param = "%{$_POST['user']}%";
$stmt = $db->prepare("SELECT id,username FROM users WHERE username LIKE ?");
$stmt->bind_param("s", $param);
$stmt->execute();

$stmt->bind_result($id,$username);
while ($stmt->fetch()) {
  echo "Id: {$id}, Username: {$username}";
}

I got the answer directly from the manual here and here.