first thing is format of private key should be like that

$privateKey = <<<EOD

-----BEGIN EC PRIVATE KEY-----

<your key here>

-----END PRIVATE KEY-----

EOD;

second thing wrong string being signed correct one :

$bodyJson = json_encode(...);

$bodyHashBinary = hash('sha256', $bodyJson, true);

$bodyHashBase64 = base64_encode($bodyHashBinary);

$messageToSign = $date . ':' . $bodyHashBase64 . ':' . $urlSubpath;

third thing invalid date format correct

$date = gmdate("Y-m-d\TH:i:s\Z");