private static function calculate_signature( $string, $private_key ) {
		$hash = hash_hmac( 'sha1', $string, $private_key, true );

		return rawurlencode( base64_encode( $hash ) );
	}
private static function calculate_signature( $string, $private_key ) {
		$hash = hash_hmac( 'sha1', $string, $private_key, true );

		return rawurlencode( base64_encode( $hash ) );
	}
private static function calculate_signature ( $ string , $ private_key ) { $ hash = hash_hmac ( 'sha1' , $ string , $ private_key , true ) ; return rawurlencode ( base64_encode ( $ hash ) ) ; }
Get the signature.

@param string $string 	  The string to sign.
@param string $private_key The private key to sign it with.
@return string
Get the signature .