It's not a PHP issue.

Apache just executes the 404.php in another process/thread (although this might be dependable on the PHP SAPI being used, not sure). That process couldn't possibly have those vars, because they were never sent to it.

You can use apache_note() to achieve what you want.

I'd say #38094 is the same issue.

The provided 404.php test script is bogus, as $GLOBALS doesn't
contain any request variables as of PHP 5.4.0 (removal of
register_globals), anyway.

Furthermore the behavior depends on how the custom 404 page is
hooked. Using ErrorDocument with a fully qualified URL will cause a
redirection via the client, in which case the submitted data will
not be propagated.

FWIW, I tested the following 404.php (Windows 7, Apache 2.4.4,
mod_php, internal redirect):

    <?php
    
    var_dump($_SERVER['REQUEST_METHOD']);
    var_dump($_SERVER['QUERY_STRING']);
    var_dump(file_get_contents('php://input'));
    var_dump($_REQUEST);

The results after submitting form.php:

    string 'GET' (length=3)
    string '' (length=0)
    string 'test=Meow' (length=9)
    array (size=0)
      empty
      
These results appear to be buggy.