Follow the steps below to generate your signature:
  1. Retrieve your clientId (one which you are passing through the header X-Client-Id)
  2. Append this with CURRENT UNIX timestamp separated by a period (.)
  3. Encrypt this data using RSA encrypt with Public key you received – this is the signature.
  4. Pass this signature through the header X-Cf-Signature.
In the case of using our library, go through the libraries section. During the initialization process, you need to pass the key as a parameter. 
<?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;
  }
?>