What is the primary use of the header() function in PHP?

The header() function in PHP is primarily used to send raw HTTP headers to the client. This function is essential for controlling how the browser handles the response from the server, allowing developers to manipulate aspects like redirecting users, specifying content types, and managing caching. For instance, when using header("Location: newpage.php"), it instructs the browser to navigate to a different URL, which is a common practice in web applications to guide user flow effectively.

While other options may involve important tasks within PHP, they do not capture the core purpose of the header() function. Retrieving server information is handled by different functions like phpinfo() or using the $_SERVER superglobal. Managing session variables is done through session handling functions, and manipulating the file system is managed by file handling functions in PHP. Hence, the specificity of function capabilities clarifies why sending raw HTTP headers is the correct main use of the header() function.