If you’ve just installed the Yosemite beta on your development machine and your web environment has stopped working – as it usually does following a major OSX update – it’s likely this.
Apache has been upgraded to 2.4 from 2.2 and you’ll need to adjust your Directory directives as follows:
Order allow,deny Allow from all
Require all granted
PHP is also upgraded to version 5.5.9 (cli) (built: May 10 2014 21:37:28). I’ve not encountered any major issue there yet but it’s worth reading upgrade pages on that too. Here’s the PHP page on the changes and compatibility issues.