HTML form submit in PHP done By HTTP, this http is a protocol and designed in such way so that server and webpage can communicate each other. HTTP send data to server from webpage or get data from server to webpage by $_GET and $_POST method. But concern is why POST is good than GET. Here you can get some brief idea about that.
In PHP $_POST consider as a super global variable which is used to collect form data after submitting a form In PHP.
Why $_POST Not $_GET?
1)POST method is used for posting the data In a hidden manner, So it will secure for sending sensitive data like password and token no. etc.
2)We can post large amount of data as compare to $_GET.
3)This method supports different data types, such as string, numeric, binary, etc.
4)Parameters are not saved in web browser history.
5)This method can be used to send binary as well as ASCII data.