PHP is not a bad language (or at least not as bad as some may suggest). I had teachers that didn't even know PHP was object oriented until I told them. I've had clients that immediately distrust us when we say we are PHP developers and question us for not using chic languages and frameworks such as Django or RoR, or "enterprise and solid" languages such as Java and ASP.NET.
Facebook is built on PHP. There are plenty of solid projects that power the web like Joomla and Drupal that are used in the enterprise and governments. There are frameworks and libraries that have some of the best architectures I've seen across all languages (Symfony 2, Doctrine). PHP has the best documentation I've seen and a big community of professionals.
PHP has advanced OO features such as reflection, interfaces, let alone that PHP now supports horizontal reuse natively and cleanly through traits.
There are bad programmers and script kiddies that give PHP a bad reputation, but power the PHP community at the same time, and because it is so easy to get stuff done PHP you can often do things the wrong way, granted, but why blame the language?.
Now, to boil this down to an actual answerable question: what would be a good and solid and short and sweet argument to avoid being frowned upon and stop prejudice in one fell swoop and defend your honor when you say you are a PHP developer?.
(free cookie with teh whipped cream to those with empirical evidence of convincing someone —client or other— on the spot)
P.S.: We use Symfony, and the code ends being beautiful and maintainable
P.P.S.: Facebook is written in PHP, compiled to C++ via HipHop and deployed on Hadoop. Here: http://arstechnica.com/business/2012/04/exclusive-a-behind-the-scenes-look-at-facebook-release-engineering/1/