PHP is computer code that assists in the underlying operations of many websites on the internet. PHP code is related to the server side rather than website code. The vast majority of websites on the internet are reliant on PHP code as part of the computer code that makes up their web servers.
As new PHP versions arrive, websites become faster. A new PHP version is one of the biggest factors that increases website speed. However, new PHP versions can also cause conflicts with the code on websites and that means websites can break. Due to this, many webmasters hold off upgrading PHP. But if you leave the upgrade for too long, you run the risk of having an outdated website that is vulnerable to being hacked. As with all computer code, outdated code can have security issues that need updating.
PHP is an open-source scripting language. You can read technical details here: What is PHP? – php.net.