I use sites like CodePen and JSFiddle all the time to write JavaScript and CSS, but I've not seen a similar site for writing PHP applications. I have used 3v4l.org to test small scripts using different versions of PHP, but it doesn't allow dependencies to be added and doesn't have a nice editing interface.
Recently, I found a site called PHPSandbox that allows users to create PHP applications that can be run in the same way as applications on CodePen. This includes using composer to inject dependencies into the application and a nice little editor interface to see the output and logs from the application.
When creating regular expressions for PHP I tend to use the awesome tool PHP Live Regex.
This tool has really helped me over the last couple of years to create regular expressions for many of the projects I've been working on. Everything from validation functions to formatters that use regular expressions have had their expressions worked out using this tool. I even use it for testing one off expressions where I just need to find/replace in a file.
Planet PHP is a feed aggregator blog specifically for PHP related blogs. There are a good number of blogs that the site uses to update with so there is always plenty of stuff going up on the site. The site is not run by PHP, and is an independent project run by Christian Stocker and Tobias Schlitt.
The good thing about this site is that it saves having to go through all of the blogs in turn to see what is interesting in the world of PHP. If you run a PHP blog you can submit your site to the list and get your articles included in the updates.
Practical PHP Programming is a wiki site that has been set up to provide a central area for PHP developers to contribute to learning and understanding PHP. This site was recently called Hudzilla and was a pretty good online PHP book.Practical PHP Programming was a pretty good site when it was just static pages, people were always linking to it from the php.net site in order to explain something that wasn't clear in the documentation. I think that the decision to turn it into a Wiki can only lead to it becoming a better resource.
Practical PHP Programming was a pretty good site when it was just static pages, people were always linking to it from the php.net site in order to explain something that wasn't clear in the documentation. I think that the decision to turn it into a Wiki can only lead to it becoming a better resource.