PHP is evolving steadily. Every year, there is a major new release containing new features, performance improvements, a fair share of deprecations, and even syntax changes. PHP core developers maintain the two latest PHP versions with active bug fixes and security fixes, followed by security fixes. This effectively means that each major PHP version will be supported at most for three years, and existing PHP applications are forced to upgrade.
While updating existing PHP applications is the ideal and recommended approach, inevitably, there are some applications/websites that cannot justify the human, political, and financial cost of the update. This is especially the case for legacy PHP applications that run on PHP 5 series or PHP 7 series. WordPress.org, for example, reports that only 16% of the reported WordPress sites run on a PHP version supported by the PHP core developers. Updating a PHP application to be compatible with the latest PHP version is on a wide spectrum of difficulties. This can range from requiring no or little changes to what feels like a complete rewrite. PHP applications that were developed over a decade ago pose the biggest challenge because they tend to use PHP extensions that are no longer supported, have no type support, and often have no automated tests to verify the changes either. Tools such as Rector can automate some, if not most, of the changes necessary, but extremely old PHP versions tend to require a lot of manual code updates.