Nonsense. Learning Python helped my JavaScript and PHP, and learning JavaScript in turn has helped my Python. Learning Erlang has helped both. PHP didn't help anything.
By making you approach the same problems in different ways, new languages make you a more versatile programmer less tied to a particular technology while improving existing skills.
Upvoted for "PHP didn't help anything". I'm pretty pragmatic about language selection (I've used Perl, Python, Tcl, C, C++, sh, and probably others over the years, and all have had their nice qualities), but I've been lightly working in PHP now for a couple of years for maintaining our company website, and I've never once thought, "Hey, that's neat! I wonder if I could do that in JavaScript/Perl/Python/Ruby."
It's a subtle but very depressing aspect of working in PHP. I just feel sad working in PHP, in a way that I've perhaps never experienced in any other language. Maybe I'm just getting too old to read more crappy code, and it would improve if I started working on better written code. And maybe I was lucky enough to mostly work with good code in the past in those other languages.
To be fair, there are some things about PHP that are surprisingly good and worth looking to for inspiration: Ease of app deployment, marketing for a "good enough" product, allowing a very large community to contribute without muddying the waters too much (comments on doc pages, for example, rather than a wiki), solving the right problems at the right time (even poorly), etc. None of these are aspects of the language itself, only the resulting full experience that is PHP.
By making you approach the same problems in different ways, new languages make you a more versatile programmer less tied to a particular technology while improving existing skills.