User names and passwords in PHP can be encrypted using md5 function.
MD5 function calculates the md5 hash of a string. It is basically used for encryption. It is also used for digital signature applications, where a large file must be "compressed" in a secure manner.
Crypt() function can also be used to encrypt a string,. It used MD5, DES or blow fish algorithms for encryption.
Salt is an optional parameter used to increase the number of characters encoded, to make the encoding more secure