The PHP Framework Interop Group has released a series of coding standard recommendations:
FIG's recommendations are far from an official standard, and I wouldn't even consider them a de facto standard, but they are the more coherent set of recommendations for PHP we have.
Other than FIG's recommendations, you should take a look at Zend Framework's standards, which include coding style recommendations, and PEAR's standards. Personally I tend to favour FIG's recommendations, they are a bit more coherent than every other set of PHP standards I've seen and are more widely adopted.
You should also take a look at PHP CodeSniffer. It's a tool that detects violations of a defined (and configurable) set of coding standards and can be easily automated via Phing. If you decide to follow FIG's recommendations, you could also use PHP-CS-Fixer to rewrite your code to be PSR-1 and PSR-2 compliant (with a pre-commit hook, for example).
Lastly, keep in mind that the main reason for adopting a coding style standard is consistency, you'll never find a set of standards that will make everyone in your team happy. It doesn't really matter which set of standards you'll pick, what matters is that you pick one and stick with it.
Yes, there are couple of standards used in the PHP world. Two of them deserve special recomendation.
The PEAR standard, which should be used by all PEAR packages: http://pear.php.net/manual/nl/standards.php
and the PSR standard, which is supported by some big PHP groups like Symfony, PEAR(again), Drupal, Joomla etc. https://github.com/php-fig/fig-standards