There are numerous algorithms listed in the PHP documentation:
How strong is each one and what criteria to use when picking one?
Also the mcrypt_encrypt() function requires
mode to be passed. From their docs:
One of the
MCRYPT_MODE_modenameconstants, or one of the following strings: "ecb", "cbc", "cfb", "ofb", "nofb" or "stream".
Again, what criteria to use on choosing a mode?