&gt; legacy apps that were built in the PHP 5.2 or even PHP 4 days, be afraid. Be very afraid<p>BS. Most PHP4 code runs just fine, as do PHP5 code. Almost no one used object oriented code in PHP4 days (it was slow!), so the incompatible changes aren&#x27;t a problem. And all you need to do is to search and replace some function names like mysql_* to mysqli_*. Most old code bases from PHP4 days also didn&#x27;t rely on frameworks at all and were completely written from scratch or copy&amp;pasted together - actually an advantage in this case.<p>Upgrading an old Python or Ruby code from 1.x&#x2F;2.x days is a lot of more work (as it always includes upgrading to a recent framework version) than upgrading from PHP4 or PHP5 to PHP7.