What algorithm does PHP openssl_private_encrypt use?

openssl_encrypt/decrypt functions specify a "cipher_algo" paramater wherein a 
specific algorithm may be chosen/specified.

openssl_private/public/encrypt/decrypt, however have no algorithm paramater so 
what do they use for an algorithm? How does one encrypt/decrypt with just a 
(real) key? Do they just "whack the data N times with the 'key'" without using 
any particular algorithm? Or maybe a hard-coded passage through an "xor tree"?