The official PHP.net has posted their PHP 7 migration guide for those already on PHP 5.6.x and wanting to prepare their applications for PHP7.

Despite the fact that PHP 7.0 is a new major version, efforts were put in to make migration as painless as possible. This release focusses mainly on removing functionality deprecated in previous versions and improving language consistency. There are a few incompatibilities and new features that should be considered, and code should be tested before switching PHP versions in production environments.

The guide includes links to other pages showing things like:

• Backward incompatible changes
• New features
• Deprecated features in PHP 7.0.x
• New functions/classes/interfaces/global constants
• Removed Extensions and SAPIs

There's also a link to some other various changes that's not completely fleshed out yet, but is evolving as PHP 7 gets closer to a final release.