In PHP 7, there are some backwards incompatible changes, so applications built with PHP 5. x that use functions and language constructs that have been removed or have the internal implementation changed drastically will spit out errors while trying to run on PHP 7.

You can learn more about this in PHP benchmarks. This means that there is an issue with backwards compatibility if you implement PHP 8.0 union types, as this will break on sites running PHP 7.4 or below.

Code that used the old right-to-left evaluation order must be rewritten to explicitly use that evaluation order with curly braces (see the above middle column). This will make the code both forwards compatible with PHP 7. x and backwards compatible with PHP 5. x.