Define constants How to prevent the address bar from accessing background files directly

Define constants How to prevent the address bar from accessing background files directly
Define is not cross-page, but recently read an open source PHP project It is this: such as a.php: direct define (' In_tm ', TRUE);
b.php Direct if (!defined (' In_tm '))
{
Exit (' Access Denied ');
}
if ($_session["admin"]!= "Silence")
{
Include template (' login.htm ');
Exit
}。。。。。
It's totally useless. Include included! But when I do this simulation, define is not cross-page! Does it have any settings that make ' In_tm ' work on another page?
Similar to Discuz to prevent the address bar directly access to the background file is how to implement it?

------Solution--------------------
If you can confirm that no files have been explicitly embedded before judging.
So he's using the Auto_prepend_file in php.ini.

In fact, as long as the project can run, you only need to print out the results of get_included_files to know what files are embedded

------Solution--------------------
This must be useful include included, just you don't see
------Solution--------------------
But when I do this simulation, define is not cross-page! Does it have any settings that make ' In_tm ' work on another page?

b.php
if (!defined (' In_tm '))
{
Exit (' Access Denied ');
}

a.php
Define (' In_tm ', TRUE);
Include ' b.php ';

Access b.php must be completed by accessing a.php, otherwise access Denied

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