is_a is deprected on PHP 5.2.x

[robneu]

Due to some ridiculousness in the PHP version history, the is_a function is deprecated from 5.0 until 5.3 when they un-deprecated it. 😕

Thanks to this, users still on PHP 5.2x would trigger some warnings when running the updater. It also looks like instanceof is a little better on performance anyway, so it's probably worth swapping out.

PR inbound.

[johnbillion]

Does External Update API even work on PHP 5.2? I've never tested it.

[robneu]

It does. Well, at least I'm pretty sure it does.... I'm using a slightly modified fork of it for some internal stuff at WP Site Care and some of our customers are (sadly) still on 5.2.