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
curl: (1) Received HTTP/0.9 when not allowed
Add the --http0.9 option.