404-header ("http/1.1404 notfound"); why not jump? (PHP)

As far as I know, add the following sentence to the. htaccess or apache config file: ErrorDocument404missing.html (note: After I have created this file, this missing.html page will appear when we input a nonexistent URL. Then I tried again. in te... as far as I know, add the following sentence to the. htaccess or apache config file:
ErrorDocument 404 /missing.html(Note: I have created this file)
That is why this missing.html page appears when we enter an nonexistent URL.
Then I tried again and wrote the following code in test. php:

The missing.html page is not displayed.
So I want to ask, how can I display it?
Or, is it wrong for me to write this? Why?
Actuallyheader('HTTP/1.1 404 Not Found');What is the purpose?

Beginner in PHP. hope you can give me some advice. thank you!

Reply content:

As far as I know, add the following sentence to the. htaccess or apache config file:
ErrorDocument 404 /missing.html(Note: I have created this file)
That is why this missing.html page appears when we enter an nonexistent URL.
Then I tried again and wrote the following code in test. php:

The missing.html page is not displayed.
So I want to ask, how can I display it?
Or, is it wrong for me to write this? Why?
Actuallyheader('HTTP/1.1 404 Not Found');What is the purpose?

Beginner in PHP. hope you can give me some advice. thank you!

HTTP status code: http://baike.sogou.com/v78674...

In Apache configuration, ErrorDocument displays the corresponding error status information on a page rather than redirection. Use 302,301 or JS, HTML meta refresh for redirect

Web server configuration