After skipping a version number (PHP 6 never saw the light of day), PHP 7.0 was released in December 2015. A year later, version 7.1 came out. The 7-series came with even more advantages compared to PHP 5. The fact that less memory was used but the performance was much better, meant that many users quickly switched to the new version. In addition to the core, the parser, lexer, and bytecode generator had also been thoroughly revised.
But good performance is not the only reason why many users like to stick with outdated PHP versions. While some simply breeze through the change, others worry about destroying their entire web presence during the changeover. There’s always a risk: it could be that after changing to a new PHP version, the rest of the code has to be adapted and plug-ins may not work as well as usual so you have to find new ways to implement features. Most of the time, however, the migration effort is much less than many web developers assume - especially within the same version. Larger changes are next expected with PHP 8. This new version should start after PHP 7.4 and is planned for 2020 or 2021.