Linux / PHP and passwords -( md5 function)

Hello all I am trying to generate a MD5 string equal to a known passwords MD5 string , with no luck in order to make a new password file i have tried use php's : $value = md5($string); and $value = crypt($string,$salt); the passwds in my systems /etc/shadow all start with $1 which i believe is standard for MD5 passwords of 32 length any one have any ideas ? Thanks in advance Neil M