I'm not sure what "old school PHP" is, since I'm only 3 years in the industry. But what I can tell you, is that when I learned OOP - first in AS3, and then in PHP - I was able to pick up other languages MUCH quicker.

I'm not entirely certain, but I can't think of a single disadvantage to learning OOP. I'd actually be interested to know if there where some.

I'd also add.. Learning PHP as a first language slowed me down a bit when exploring other languages. PHP is very forgiving. If you're just learning to learn.. Maybe you should consider starting with a language that's much more strict; Python or Perl in my case. This way you focus on the concepts instead of the practices. PHP is one of those.. "more than one way to skin a cat" languages. Or at least more than most (from my experience).