I'll add my $.02. I have a pretty advanced programming background and find that once you know how to program, its just syntax to pick up 80% of a language (the other 20% comes from learning the nuances of the language itself, how to do things more quickly with builtins, etc).
I can't speak for Ruby because I don't know a line of it. I do know PHP very well and did some large scale web development (> 20k lines of custom code) on a contract a couple of years ago. Its handy for the web, but not much for anything else (despite the fact that a PHP GTK module exists among others). I'd stay away from it if it is going to be your only strong language. OTOH, if you already have another language and want to sell yourself as being able to deliver web solutions to problems as well, then maybe its the ticket.
I'm a Perl person when it comes to doing admin stuff. I find they have a great user community at perlmonks.org. For three years, I prototyped most of my code in perl (for proof of concept/rapid deployment of a stopgap solution) before writing a final product in C++. As you know, sometimes the software life cycle doesn't work out and you never get around to a final solution. Some of my perl code is still out there and has scaled VERY well.
I'm just now having to learn Python to incorporate functionality into a client's existing application suite that is written in python. I can't speak for it either.
I agree with Andrew that you should definitely learn with a project rather than for learning's sake. That's been my key to learning anything was to put it into application. There are plenty of open source projects out there that need help, even scripting help. If you are looking to learn the language for a resume builder, get involved withe one of the projects and contribute while learning the language or build an add-on (like ndiff for nmap). Pick the project you contribute to by the community you want to work in and possibly by the time you are job searching your technical interviewer will have heard of (or used) the project you contributed to. That's a resume builder.
Certifications: CREA, MCSE: Security, CCNA, Security+, other junk