Watch out for the new default charset (selecting charset may not be anything you can do with your Internet service provider). Having a lot of old databases with latin1 encoding generates work with the upgrade.
Most improvements in PHP 5.6.x have no impact on existing code. 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 and 5.5.x.
Watch out for the new default charset (selecting charset may not be anything you can do with your Internet service provider). Having a lot of old databases with latin1 encoding generates work with the upgrade.