$dbc = mysqli_connect($servername, $username, $password, $database);
$sql = "INSERT INTO signup (username) VALUES ('".$username?$username:'Упс кажется параметр пуст'."')";
if (mysqli_query($conn, $sql)) {
//return
} else {
echo "Error: " . $sql . "<br>" . mysqli_error($conn);
}
mysqli_close($dbc);