Despite the fact that PHP 7.0 is a new major version, efforts have been made 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.
See also the migration guides for PHP versions 5.0.x, 5.1.x, 5.2.x, 5.3.x, 5.4.x, 5.5.x and 5.6.x.