php $_GET and $_POST functions
PHP provides two special functions called $_GET & $_POST, this functions are used to get the values which are passed from a user-filled form, like user registration or login form etc.
XML Expat Library:
PHP's XML parser is based on Expat. It lets you parse but not validate XML documents
means it does not allow us to find out the XML tags which are present instead of that we can check the correctness of the structure.