PHP (Hypertext Preprocessor) is a widely-used open-source server-side scripting language designed primarily for web development. It is embedded within HTML code and executed on a web server, generating dynamic web content that can be sent to the user's browser for display. PHP scripts are commonly used to create dynamic web pages, process form data, interact with databases, and perform various server-side tasks.

Key features of PHP include:

1. Easy Integration with HTML: PHP code is embedded within HTML, making it easy to mix dynamic content with static content in web pages.
2. Open Source: PHP is open-source and freely available, which has contributed to its widespread adoption and large developer community.
3. Cross-Platform Compatibility: PHP can run on various operating systems, including Windows, Linux, macOS, and more, making it versatile for different server environments.
4. Database Connectivity: PHP provides built-in support for connecting to various databases, such as MySQL, PostgreSQL, SQLite, and more. This enables developers to create dynamic web applications that interact with databases to store and retrieve data.
5. Server-Side Scripting: PHP is executed on the web server, not in the user's browser. This allows developers to perform tasks on the server side, such as processing form submissions, authenticating users, and managing sessions.
6. Large Community and Documentation: PHP has a large and active developer community, resulting in a vast amount of documentation, tutorials, and libraries available for various tasks.
7. Extensibility: PHP can be extended using various libraries and frameworks, such as Laravel, Symfony, and WordPress, to simplify and accelerate web development.
8. Wide Adoption: PHP has been used to build a significant portion of the web, from small personal websites to large-scale applications like social media platforms and e-commerce sites.
9. Security Measures: While security concerns have historically been associated with PHP, improvements have been made over time to address vulnerabilities and enhance security features.
10. Fast Execution: PHP's interpreted nature allows for relatively fast development cycles, and with optimizations, it can perform well for a variety of web applications.

It's worth noting that as of my last knowledge update in September 2021, PHP continues to be relevant in web development, but the landscape might have evolved further since then. Always refer to the latest sources for the most up-to-date information.