crypt() will return an encrypted string using the standard Unix DES encryption method. Arguments are a string to be encrypted and an optional two-character salt string to base the encryption on. See the Unix man page for your crypt function for more information.
If the salt argument is not provided, one will be randomly generated by PHP.
Some operating systems support more than one type of encryption. In fact, sometimes the standard DES encryption is replaced by an MD5 based encryption algorithm. The encryption type is triggered by the salt argument. At install time, PHP determines the capabilities of the crypt function and will accept salts for other encryption types. If no salt is provided, PHP will auto-generate a standard 2-character DES salt by default, unless the default encryption type on the system is MD5, in which case a random MD5-compatible salt is generated. PHP sets a constant named CRYPT_SALT_LENGTH which tells you whether a regular 2-character salt applies to your system or the longer 12-char MD5 salt is applicable.
If you are using the supplied salt, you should be aware that the salt is generated once. If you are calling this function recursively, this may impact both appearance and, to a certain extent, security.
The standard DES encryption crypt() contains the salt as the first two characters of the output.
On systems where the crypt() function supports multiple encryption types, the following constants are set to 0 or 1 depending on whether the given type is available:
CRYPT_STD_DES - Standard DES encryption with a 2-char SALT
CRYPT_EXT_DES - Extended DES encryption with a 9-char SALT
CRYPT_MD5 - MD5 encryption with a 12-char SALT starting with $1$
CRYPT_BLOWFISH - Extended DES encryption with a 16-char SALT starting with $2$
There is no decrypt function, since crypt() uses a one-way algorithm.
See also: md5().