PHP is a server-side scripting language that can be embedded into HTML pages using PHP tags. When a PHP page is requested, the server will execute any PHP code and output the results. PHP allows variables, control structures, and functions to handle tasks like form processing, file uploads, and database access. Functions like file() can read file contents into an array, and files can be uploaded and moved using the $_FILES array and move_uploaded_file() function. PHP scripts can generate dynamic web page content on the server before sending the page to the client.