Add an user to the AuthDigestFile.
$user and $realm must start with an alphabetical charachter and must NOT contain any other characters than alphanumerics, the underline and dash.
the user to add
the realm the user should be in
the plaintext password
Returns TRUE on success, PEAR_Error on failure.
Table 43-1. Possible PEAR_Error values
|FILE_PASSWD_E_EXISTS_ALREADY||user already exists in the supplied realm|
|FILE_PASSWD_E_INVLAID_CHARS||user or realm contains illegal characters|