In this new post on thePHP.cc blog Stefan Priebsch talks about some of the backwards compatibility breaks that will be coming with PHP's next major version, PHP7.
According to the PHP project's current time line, PHP 7 is scheduled to be released later this year. The version number 6 will be skipped for good reasons. As is expected of a new major release, there will be some breaks in backwards compatibility. Such breaks are always a double-edged sword: some have been eagerly awaiting the removal of legacy features, others expect that existing software keeps working without modifications. The PHP project is notorious for keeping some sins of the past dating back to PHP 3 in an effort to ensure backwards compatibility. Now, with the release of PHP 7, the decision has been made to remove some features that have been marked as "deprecated" in PHP 5.
He talks about how PHP will be "re-engineered" for this major release including a uniform variable syntax and some of the things this could break (like Magento 1). He also mentions the removal of the mysql (not mysqli) extension and a major issue - that PEAR has stopped working in recent versions of PHP7 (built from the current codebase) because of how it calls non-static methods statically.