What is it?
A general purpose scripting language.
"PHP is a server-side scripting language designed primarily for web development but also used as a general-purpose programming language. PHP originally stood for Personal Home Page, but it now stands for the recursive acronym PHP: Hypertext Preprocessor". Source: Wikipedia.
PHP powers your website.
"Each release branch of PHP is fully supported for two years from its initial stable release. During this period, bugs and security issues that have been reported are fixed and are released in regular point releases.
After this two year period of active support, each branch is then supported for an additional year for critical security issues only."
Check whether latest PHP release is a Security Fix
It is not immediately obvious whether or not the latest release of PHP is a Security Fix when you read the Change Log for any given release of PHP.
One factor to consider when upgrading PHP is that the PHP Module Sets change slightly from one version to another.
Some 3rd party Joomla! extensions require specific PHP Modules to be installed for them to work.
If after changing to a later version of PHP you see a warning next time you visit your website, check your server's PHP settings.
PHP settings can usually be adjusted in your Hosting Control Panel.
For example, cPanel includes PHP Switcher which enables you to tweak the Module Set for any one version of PHP.
It may however be necessary to contact your hosting company's Technical Support.