Those who have the slightest experience of Ruby on Rails would understand the importance of MVC (Model View Controller) framework for developing web applications. RoR is awesome, there is no doubt at all. But as a web developer, I have to build websites for others and this means I have to use their web host. Unfortunately, there are a very few web hosts supporting Ruby (compared to PHP).
When it comes to MVC framework in PHP, you would immediately think of CakePHP, CodeIgniter, Symfony or Zend Framework. Now the question is which one serves the purpose best for you.
If you want your application to run in most servers, then you cant rely on Symfony much since it requires PHP5 while CakePHP and CodeIgniter both run in PHP4.4.x to PHP5. If you want search engine friendly urls, then you will require mod_rewrite module for apache (not a must though - you can still run it without mod_rewrite).
To be fair, CodeIgniter is more organized in terms of documentation, and CakePHP is still updating its Cookbook (for version 1.2 beta). They have Screencasts that beginners will find useful. Most importantly, their community is very strong, and you will get quick response if you get into any problem.
CakePHP has a pretty active community as well. They have the Bakery where users can participate and submit articles, code snippets, etc. Moreover, there is a Google Group and IRC channel where you can ask for help anytime.
I am a big fan of CakePHP, and will stick to it. According to Google Trends, there are more people searching for CakePHP than any other popular php frameworks. I will always support going for that open source project that has a huge community (or has the potential).