about 404 page

[wpl12]

I have changed the 404 page to reply.php page,in this page,
There is 2 question.
First , the url cant set "/reply.php" ,could set "http://domain/reply.php",
Second ,when the url set "http://domain/repy.php",the page can use
$_SERVER['HTTP_REFERER'];It's always show nothing.

[mistwang]

Why 404 page cannot be set to "/reply.php"?

When set to "http://domain/reply.php", it becomes a external redirect with status code 302 will be returned, I think "Referer" header has not been sent by the browser.