question about direct access to url

my question is about displaying a friendly message when someone directly types a url in the browser. example i have one file

called form.php which processes the information entered in the form and lets say the next page is
thankyou.php?firstname=david by seeing the url if someone types thankyou.php?firstname=smith in the address bar the browser

will display smith.

1. so if a user enters a url in the browser directly to a specific page("thankyou.php") i would like to display a message

like = "Direct access to this file is not allowed"

2. with the above method assuming even if i type thankyou.php in the browser directly, though i own the file i will also get
the message "Direct access to this file is not allowed". so i would not like to see this message, i guess for this i need to

specify my IP address i suppose.

3. also if a user by mistake types the wrong url directly in the address bar ex= www.website.com/abou.php instead of
typing www.website.com/aboutus.php instead of the browser displaying File not found or a 404 error message i would like to
display a customized page which will still have the same look and feel of my website in terms of the layout and i would like

to display a message such as = "Page could not be found" Does this have to be done from apache perspective or can it be done
using php. please suggest the procedure in either case apache OR php.

please advice.

thanks.

 

1 & 2

<?php

$ip = "127.0.0.1"; # change to your ip

if($_GET['firstname']==""&& $_SERVER['REMOTE_ADDR']!=$ip) {
	echo "Direct Access To This File is Not Allowed";
} else {
	#show page here
}

?>

PHP:

3 Create a .htaccess file in the root and put

ErrorDocument 404 /custompage.html

Change /custompage.html to whatever error page you want.

 

thanks for providing the 404 method. it worked.