Files with the extension PHP3 are Hypertext PreProcessor Script files in version 3.
PHP is a scripting language that runs on a web server, producing output in the form of HTML pages. PHP has many features that simplify collaboration with databases such as MySQL and allow you to build rich web applications such as content management systems.
Since PHP is an open source project, it has become very widespread and is used on many websites. Applications range from simple forms to complex management systems and shops.
Files with the extension PHP3 can contain pure PHP source code as well as this embedded in an HTML. It is similar to ASP, CGI or JSP.
To edit a PHP3 file, just use a text editor. But there are also complete development environments that greatly simplify the development of PHP scripts.
The extension PHP3 or php in a URL (in the address line of a browser) indicates that PHP is being used on the website. The user of the website receives only the PHP script generated HTML document, not the actual PHP script.
The usual extension for PHP files is php. The version in the extension is not obligatory. However, if used for referencing, it must of course persist.