Change linux password

Maybe this question is not 100% PHP, but it could be. How do I with Apache and PHP 4+ running under RedHat 5.0, change a users password in /etc/passwd (shadow)? And I would like to do it without running the web server as root. Can PHP force a process to run as superuser somehow and in that way be able to execute the passwd command to change password, or are there another command that might be more helpful? If I don't remember wrong this issue have been discussed somewhere around here before, but it's too long ago for me to remember. Thanks! --- Johan Björk