PHP7 was released last week! This new release has some breaking changes and new features, and isn't entirely backwards compatible with PHP 5.x.
Unfortunately, MODX Revolution 2.4.2 does not currently work on PHP7. There are fatal errors, caused by those breaking changes, in the manager, and some features like thumbnails aren't quite working.
The good news is that over the past few weeks there have been a number of community members that have been trying MODX on PHP7, and have been reporting issues and sending many pull requests that resolve specific issues. These are currently getting merged into the next releases of MODX, meaning that soon you'll have no problem running MODX on the latest and greatest PHP version.
Some fixes - the most urgent ones - have been added into 2.4.3, however full PHP7 support should be accomplished in 2.5.0. Given these PHP7 issues and the number of fixes and improvements that are available on git, I think we'll see both 2.4.3 and the first 2.5.0 pre-release before the end of the year or shortly into 2016, so the releases aren't far out.
If you'd like to help get MODX ready for PHP7, you can install MODX from git (use the
2.x branch) and provide bug reports and/or pull requests if you find any issues. Third party extras will also need to be tested, as it is possible for extras to contain code that wont work in PHP7 as well.
Want to learn more about PHP 7? You can check out the migration docs, watch some screencasts explaining the new features or read what Zend has to say about it.