There are numerous algorithms listed in the PHP documentation:
http://php.net/manual/en/mcrypt.ciphers.php
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_modename
constants, or one of the following strings: "ecb", "cbc", "cfb", "ofb", "nofb" or "stream".
Again, what criteria to use on choosing a mode?