While this method does work for me, I do get broken links (where images are missing) on those error pages. Not sure what's causing that. Maybe my occasional upload of updates... kinda a drag to have to keep opening up a new project and uploading the error pages from there.
@wwonderfull had suggested the 'html error page' way of doing this.. I'm now wondering if that may be less of a hassle.
404 error page is very simple to know.
In reality I my self as a user rarely got 5 views of 404 error page in a hole year of website browsing. Because the website is removed or has technical problems or the page has been removed from websites host server and showing on google's search engine result for some days before termination. 404 error page was just for a simple indication for people to know that the server cannot find the requested resource. Some people use searches and all that depending on what kind of content they have if they have hundreds and thousands of page they can use searches on 404 error page to lead to another page. But as I said it was an indication for people to know that the server cannot find the requested resource.
So just to show that "page not found" line people made simplest of way to show it from html to php. its just a basic text in the corner saying page not found. But now in modern days people added visuals backgrounds animations and more advanced use of it. It really depends on you what you want people to do when people find your broken page. If you want you can redirect a 404 error page user back to home page or the new page you have created.
HTML or PHP the main purpose is to show people that the "server cannot find the requested resource". So that being known, now I think you can decide what you would like to do with an 404 error page. You can use it creatively or you can use it natively meaning classic.