Restrict Direct Access to some pages

I need to restict direct access to one page (lets say "pageB.php"
)inside my website, so the only way to reach it were clicking a link
in the main page (lets say "pageA.php").

If someone tries to go directly to "mysite.com/pageB.php" I need to
show an error message saying "Wrong Entry" or something like that.

Any clue?

Thanks.