Novice ask legitimate user to verify related issues

Novice Consult legitimate user authentication problem
Hello everyone! I have recently started to learn the PHP landing interface is this meaning if the account and password as the database so give a session= user name

Then at each interface to detect whether legitimate users with include check.php

The main idea of the code is that if the session is empty then prompt illegal but found that the security is very poor what are the good ideas and code?

The novice doesn't have much to be ashamed of
------Solution--------------------
$_session["admin_name]==" "and session==" "not the same.
------Solution--------------------
Session_Start ();
$_session[' username ']= "123";
PHP program, not form can be submitted!
Of course, if someone goes to your server and modifies your program, it's another matter.

