PHP (Hypertext Preprocessor) is a popular scripting language used by many website design companies for the development of dynamic and interactive websites. PHP allows developers to create dynamic pages, add features such as user login and registration, e-commerce functionality, and database integration.

Some of the benefits of using PHP in website development include:

1. Open Source: PHP is an open-source scripting language, making it free for developers to use and easy to access for website design companies.

2. Easy to Learn: PHP is easy to learn, making it ideal for website design companies to incorporate into their development process.

3. Server-Side Language: PHP is a server-side language, meaning it runs on the server and outputs HTML code to the browser. This helps with the security and speed of the website.

4. Integrates with Databases: PHP integrates easily with databases such as MySQL, making it easy for developers to add database functionality to their websites.

5. Supports Various Platforms: PHP can run on various platforms such as Windows, Linux, and Mac, making it a versatile choice for website design companies.

Overall, PHP is a widely used and effective scripting language for website design companies, allowing them to create dynamic and interactive websites.