I wanted to play with Symfony 2 and so needed to install PHP 5.3 on my OSX 10.5. What seemed like a simple task turned into a huge waste of my time. I use MAMP because it's a nice out-of-the-box solution. The problem is, it still doesn't ship with 5.3 because it's waiting for it to be "stable".
She tried to find a way to upgrade MAMP's installation to the latest stable version of PHP but was stopped by dependency errors and opted to go with XAMPP instead to get more of what she needed. She also has included a few edits to the post with more information - how to migrate your virtual hosts from MAMP to XAMPP and migrating over the MySQL structure.