In a recent developer vote, the php project decided to deprecate the mysql extension in PHP 5.5 and finally remove it from the main PHP project. It may or may not be available for a longer period as a PECL extension.
The mysql extension has long been superseded by two more powerful extensions, PDO/Mysql and mysqli (improved). For years, the older extension has not received any new features and the developers kept it around just to keep compatibility with old code. Framework and application developers are now urged to update their code to use one of the alternative mysql APIs. There are a lot of old code snippets and tutorials around which describe the old API. Eventually, this code will begin to throw warnings and finally stop to work.
Developers discussed the impact of this move on end users. While it might be shocking to see hordes of old installations break just because the hoster updates his PHP version, there is no need to panic. Most hosters have not even upgraded to the recent PHP 5.4 release and it might be years to go until PHP 5.6 finally hits enterprise distributions like SLES or RHEL. Additionally, distributors and hosters might opt to provide the PECL version of the mysql extension for backward compatibility. There is enough time left for developers and end users to react on the coming change.