PHP mysqli_query not showing up in database

I am trying to input data into a database with PHP and mysqli. There is no error and the code runs, but when I look in the database, it says that a row has been added but the columns "usernames" and "passwords" are empty.

$sql = "INSERT INTO $table (usernames, passwords) VALUES ('$username','$password')";
if(mysqli_query($db, $sql)) {
    echo "new user successfully created";
} else {
echo "Error: " . $sql . "<br>" . mysqli_error($db);
}

I think there is something wrong above but I am not sure what it is.

EDIT: I think the problem is with $username and $password not having any value. It might have to do with the following code:

    if ($_SERVER["REQUEST_METHOD"] == "POST") { //remove whitespace, slashes, and unnecessary characters
    $username = test_input($_POST["username"]);
    $password = test_input($_POST["password"]);
}

function test_input($data) { //function to remove unnecessary characters
    if($data != trim($data)) {
        die("error: there is an unnacceptable character in your username or password");
    }
    if($data != stripslashes($data)) {
        die("error: you cannot have slashes");
    }
    if($data != htmlspecialchars($data)) {
        die("error: password not accepted");
    }
}