The error in your code is caused by the argument in the mysqli_connect_errno() function. The function does not accept any arguments. However, in your code, you are passing "$connection" as an argument to it.
Remove the argument from the function and it will fix the error.
Here is the modified code:
if (mysqli_connect_errno()) {
echo "Could not connect to DB: <br />" . mysqli_connect_error();
}