Please excuse the subjective nature of this question but I'm trying to determine whether to focus on PHP for my next job search because I'm looking for a less formal work environment.
Do I have an impression that PHP jobs tend to be in less formal work environments such as smaller companies/startups with more of a progressive attitude?
My current position as a PHP Developer is for a smaller organization (Non-profit). But my last position was for a multi-million dollar mid-sized company that was developing rather impressive solutions with nearly all open-source/free technologies.
I do think, and I may be wrong, that the larger the company the greater the chance they're using something like .NET in the corporate world. That's based on me having two jobs for larger companies, and both asking that I use .NET solutions. Plus meeting several friends in user groups who work for larger companies, which also use .NET. I blame this on Businessmen running these companies and thinking "If we pay for it, it's better." This thinking is obviously flawed.
You need to consider also that PHP/MySQL/Linux are all "Free". Smaller organizations (without the massive budgets) will naturally gravitate to these options. It doesn't mean that these are inferior options, only that they are naturally more appealing to startups and small groups.
Large corporations tend to prefer mainstream, established, conservative technologies. In my experience, that means COBOL, C, C++, Java, C#, or VB. Python, Ruby, Lisp, PHP, etc. aren't as common.
I can't say that small companies always prefer those language over Java or C#, but I can tell you that sticking with PHP will make it impossible for you to work at the larger companies that I've been a part of.
Whether a company is informal or progressive or fun to work for has little to do with what scripting language they employ.
Maybe you should explain what you do now, and what kind of situation would be 'less formal' enough to be attractive to you. PHP is everywhere now, which means you could find yourself working at a large but conservative company on interesting projects, or at a progressive startup doing boring work to meet payroll, or in some other combination of the above.
Perhaps you should try figuring out where you want to work and learn what they do.
PHP jobs will run the board from stuffed shirt corporate gigs to stylish shirt agencies to blue shirt kaki-pants, to wear any kind of shirt you want startups. The language is un-designed so that programmers from any environment will find something familiar in there, which has helped it's adoption across wide swaths of the industry.
You will find PHP in places with less money. There's no licensing needed to start working, initial deployments are dirt-cheap, and so are people claiming toe PHP programmers. These grey collar programming jobs tend to come with a less formal dress code.
Your best bet with PHP is to work for a consultancy that does Websites for small-to-medium sized companies. That can still be quite lucrative. Working directly for those same companies tends not to be. Also the smaller a company is the more likely they won't need programmers 100% of the time, making the consultancy that much more attractive (even if its more expensive).
And yes I realize that PHP is used at some big companies like Facebook, Flickr and Yahoo but they really are the exception rather than the norm. A typical large company (that isn't a "technology shop" like those three) simply won't (most of the time).