md5() built-in php function generates md5 hash of a string using MD5 Message-Digest Algorithm and returns 32-character hexadecimal hash number. It was first designed by Ron Rivest in 1991. Although md5 has function is widely used in different software applications but it has various security vulnerabilities such as Collision attack, preimage attack etc that's why MD5 is not suitable for the application of SSL certificates or digital signatures.