It is relatively impossible directly with PHP.
PHP runs as whatever user Apache runs as, usually something like \'nobody\' or \'www-data\'. This means it has no access privleges.
Especially not root access, which is required to add a user.
The only reason I could see you wanting to add system accounts is for web based email. And even then, it is risky (Giving out free system accounts = bad).
I do web based email at my work and have the person who administers users use Webmin to add users. But only he and myself can do it through Webmin.
We do however let users change their own password, via popassd and a nice PHP script I found.
