PHP its a most popular scripting language using this we can create web applications. PHP run on all popular servers like Apache and IIS servers.PHP means Hypertext Preprocessor its a open source that means we can get download with out paying amount.
If we want to learn php first we have to learn HTML. PHP file extinction should be .php only.Current php version 5.6. wordpress,drupal cms build on top of php only many CMS and Frameworkd available now a days. which are developed on top of PHP only.
If you want to execute php first we have to install server(like Apache or IIS,..) for creating the web application we have to install any one of browser.