cookie密码难理解

$str = base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_256, md5($secureKey), $str, MCRYPT_MODE_ECB));

$str = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, md5($secureKey), base64_decode($str), MCRYPT_MODE_ECB);

请问这两个加密和解密的语句中的base64_encode、mcrypt_encrypt、MCRYPT_RIJNDAEL_256、md5($secureKey)、MCRYPT_MODE_ECB、以及解密的顺序怎么理解啊？求大神帮忙！