help how to execute exec

hi ppl i am using apache on redhat linux. i am making a script of change password in qmail. for that reason i am using system and exec command to execute that command /home/vpopmail/bin/vadduser and vpasswd and that files has 755 permission with vpopmail user and vchkpw Group. when i use exec function ,it give me illegal user name system("/home/vpopmail/bin/vpasswd $SESSION_USER_NAME $new_pass") so how i do it.... if i want to use linux sudo command then i use it , any body have idea. thanks zafar