I need some help understanding the documentation on the mysqli methods in the manual. Basically, I don't get what the difference is between methods described using the scope resolution operator ::, and those using the arror ->.
Some methods can only be used separately, like mysqli_connect_error() or mysqli_debug().

In the PHP.net manual methods are shown with a double colon/scope-resolution-operator (the Paamayim Nekudotayim) and properties are shown with an arrow.

For example:
MySQLi::__construct()
MySQLi->affected_rows

I'm not sure on the reasoning for this, as I tend to think static methods when I see an SRO but that's just the way it's shown in the manual.