<?php
public static function getSignature() {
    $clientId = "<your clientId here>";
    $publicKey =
openssl_pkey_get_public(file_get_contents("/path/to/certificate/public
_key.pem"));
    $encodedData = $clientId.".".strtotime("now");
    return static::encrypt_RSA($encodedData, $publicKey);
  }
private static function encrypt_RSA($plainData, $publicKey) { if (openssl_public_encrypt($plainData, $encrypted, $publicKey,
OPENSSL_PKCS1_OAEP_PADDING))
      $encryptedData = base64_encode($encrypted);
    else return NULL;
    return $encryptedData;
  }
?>