Cannot stream request using CURLOPT_READFUNCTION without chunked encoding

Description

Consider this curl code in C:

int counter = 0;
size_t read_callback(char *ptr, size_t size, size_t nmemb, void *userdata)
{
	if (counter++ == 1) {
		return 0;
	}

	ptr[0] = 'W';
	ptr[1] = 'H';
	ptr[2] = 'A';
	ptr[3] = 'T';
	return 4;
}

int main() {
	CURL* handle = curl_easy_init(); 
	CURLcode res;
	curl_easy_setopt(handle, CURLOPT_URL, "http://localhost:9090");
	curl_easy_setopt(handle, CURLOPT_READFUNCTION, read_callback);
	curl_easy_setopt(handle, CURLOPT_POST, 1);
	curl_easy_setopt(handle, CURLOPT_POSTFIELDSIZE, 4);
	res = curl_easy_perform(handle);
	curl_easy_cleanup(handle);

	return 0;
}

That results in the following HTTP request

POST / HTTP/1.1
Host: localhost:9090
Accept: */*
Content-Length: 4
Content-Type: application/x-www-form-urlencoded
WHAT

When removing the CURLOPT_POSTFIELDSIZE options, the following request is issued:

POST /iserv/helloworld HTTP/1.1
Host: localhost:982
Accept: */*
Transfer-Encoding: chunked
Content-Type: application/x-www-form-urlencoded
Expect: 100-continue

WHAT

That is analogous to its PHP implementation:

$counter = 0;
$ch = curl_init();

curl_setopt($ch, CURLOPT_URL, 'http://localhost:9090');
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_READFUNCTION, static function ($ch, $fd, $length) use (&$counter) {
        if ($counter++ > 0) {
            return "";
        }

        return "WHAT";
});

$output = curl_exec($ch);
curl_close($ch);

However, PHP does not expose the constant CURLOPT_POSTFIELDSIZE, i.e. I have no option to tell curl the length of the request. As a result, I cannot stream a request body using CURLOPT_READFUNCTION without chunked encoding.

PHP Version

8.0.12

Operating System

Ubuntu 21.10

Start with PHP's curl option handling for CURLOPT_READFUNCTION and the missing CURLOPT_POSTFIELDSIZE binding, then compare those entry points with libcurl's corresponding options. Reproduce the request shown in the issue and verify that a streamed body can provide its length without using chunked encoding.