Why Sending a 404 Error in PHP May Not Work

In PHP, if you encounter an empty page despite using code to set a 404 Not Found header, it's because:

if (strstr($_SERVER['REQUEST_URI'],'index.php')) {
    header('HTTP/1.0 404 Not Found');
}

While technically correct, this code lacks a crucial step: PHP is responsible for outputting the 404 error page.

How 404s are Typically Handled

Normally, 404 errors are managed by the web server:

1. User requests a URI.
2. Webserver checks for the file at that URI.
3. If the file is not found, the webserver sends a 404 header and displays a 404 error page.

PHP's Role in Error Handling

However, when a PHP page is executed, the webserver passes control to PHP before it can handle a 404.

1. User requests a URI.
2. Webserver detects a PHP page and passes control to PHP.
3. PHP checks for the file and discovers it's not found.
4. PHP sends a 404 header but now becomes responsible for displaying the error page.

In this scenario, PHP lacks the capability to show a custom 404 page, resulting in a blank page. To resolve this issue, you can incorporate your desired 404 page within the PHP code or redirect to a dedicated 404 page that exists outside of the PHP application.