Send "HTTP 100 Continue" response:

$headers = getallheaders();

if (array_key_exists('Expect', $headers)) {
	// header("HTTP/1.1 100 Continue");
	 http_response_code(100);
}

Test

curl -H 'Expect: 100-continue' -v -X POST -d "" --http0.9 http://domain.tld

Add the --http0.9 option.