Bitcoin's pay-to-public-key-hash is a standard transaction that, in stead of recording the receiver public key in the blockchain, record only the ripdem160(sha256(pKey)). For spending it, one mus provide a key that it's hash is equal to the value in blockchain. The use of two different algorithms is made to reduce the chance of an attacker find the right hash using pre-image or some  attack to the hash. To broken a p2pkh you must broken both sha256 and ripdem160, as well find the right public key to create the pre-image (only a limited set o public keys on the whole space). Theoretically, if you find a good way to calculate the discrete log and use it to broke bitcoin keys, you will not go so far, because you don't know what public key look for (except for the very first transactions, and in case of key reuse)