Which PHP mcrypt cipher is safest?
So guys, there's plenty of different ciphers available - but which one is the safest to use nowadays?
Answer
If unsure use AES (also known as "Rijndael") with a 128-bit key. If you have developed some kind of fetish about key size then you could fulfill your irrational qualms by selecting a larger key, e.g. 192 or 256 bits; the extra cost is not high (+40% workload for AES-256, compared to AES-128, and it takes a very very fast network to actually observe that difference).
Beware that, regardless of the key size chosen, the correct mcrypt cipher for AES is always MCRYPT_RIJNDAEL_128. This is because the AES standard refers to the flavor of the Rijndael cipher with a 128-bit block size. If you want AES-256, you need to use MCRYPT_RIJNDAEL_128 with a 256-bit (32 byte) key, not MCRYPT_RIJNDAEL_256.
AES was published in 1998 and adopted by the US government as a federal standard in 2001, and it shows no sign of weakness nowadays. Some mathematical properties were found later on, but they do not impact actual security; mostly, they highlight that we have some relatively precise knowledge on why AES is secure. No other symmetric encryption algorithm has received as much attention (by thousands of talented cryptographers) than AES.
Most security issues come from how the cryptographic algorithm is used, not the algorithm itself. Use a proper chaining mode, add a MAC, manage padding, and most of all handle the keys securely. If you got all of this right (which is much more tricky than what it seems) then it becomes time to worry about choosing Rijndael, Twofish or whatever.