To find out how to load a file in PHP from a form you have to start by formatting the form correctly. It is mandatory to use the post method as well as the multipart/form-data enctype. otherwise PHP will not receive the necessary data.
In PHP, to recover data, you have to use $_FILES.
by doing this, we get different information
Array ( [namefile]Array ( [name]myfile.xml [type]text/xml [tmp_name]/tmp/phpCGlZPa [error]'0' [size]577059 ) )
From this moment on you can do all the necessary actions, such as saving the file on the server or parsing it like an xml.