If you use an Apache/Nginx HTTP server, it executes PHP scripts only after it finished loading the whole request from the client - which is too late for your use case, as Sergio in the other answer correctly points out.
There is a single-request solution in PHP, but you need to have control over the HTTP requests in your PHP script.
You can chose to not use Apache, but instead start a HTTP server from your php-cli (either by using the native socket functions or some HTTP server package such as react/socket that uses them in the background).
$loop = React\EventLoop\Factory::create();
$socket = new React\Socket\Server('127.0.0.1:8080', $loop);
$socket->on('connection', function (React\Socket\ConnectionInterface $connection) {
// here you can have $connection->on(...) event handlers
});
$loop->run();
Then you can have handlers that handle each chunk of the incoming request (example from the react/socket package, specifically ReadableResourceStream):
$connection->on('data', function ($chunk) {
echo $chunk;
});
And instead of echoing the chunk, you can validate its contents and call $connection->close() if you need, which effectively terminates the unfinished upload.
But this whole thing is a complex solution, and I'd recommend to use it only for a upload service that is completely separated from the application that generates the form page (which can still run under a regular Apache HTTP server because it's just much easier).
