Many PHP applications are still running on PHP 5.x, not taking full advantage of the awesome features that PHP 7 offers.
A lot of developers have not made the switch because of certain fears of compatibility issues, migration challenges, and the awkward feeling that migrating their apps will take away a big chunk of their time.
If you are looking for a way to understand PHP 7 features like a boss, and migrate that PHP 5 app without hitting your head against the wall, then this handbook is for you.