How To Execute Shell Scripts As Some User

I'm trying to write some scripts that automate the installation of a website with a content management system. This includes creating domains, installing databases etc via plesk API.

Now we have it creating the domains and databases via plesk, this is not the issue. It's permission problems issuing commands on the server.

I need to have the script install the cms and this requires writing files to a folder on the server owned by a user which is created with the domain.

e.g.
user creates timsdomain.com with ftp user name tim
Plesk makes directory on webserver called ~/vhosts/timsdomain.com/
inside this directory is a folder called httpdocs.
All the files within this folder are owned by tim.

Now my script (running as 'apache' in a folder owned by 'admin') needs to write/edit files to this directory as the user tim (or whatever user was created when the domain was set up).

How can I do this from a php script?

My idea would be to log in as the new user and then start issuing commands but I don't know how to login a user from a script.

I've tried exec, exec_shell etc
trying to log in via script as the new user using sudo/su/suid etc but they all ask for a password (for obvious reasons) but I cannot see a way to send the password to the command with the script. Ssh?

Here's some pseudocode that demonstrates what I'm trying to do, if you can think of a better way I'm all ears.

Code:
//User Fills in CMS/Domain information which becomes Form_Data

//uses plesk to create blank domain, create email addresses and empty database
Make_Domain(Form_Data) {
Plesk = Get_Plesk_Object();
Plesk.Create_Domain();
Plesk.Create_Database();
Build_CMS(Form_Data);
}

//HERE IS WHERE I AM HAVING TROUBLE: I can't figure out how to change the user so I can install files to a
// different directory other than the current user's directory (admin)

Build_CMS(Form_Data) {

change_to_user(Form_Data.ftp_username, FormData.ftp_password);
unpack_CMS_archive_into(user_directory); //eg unpack cms.tar into 'timsdomain.com' (if the above command didn't switch the current directory to users home directory)

//initialises CMS config files and installs CMS data to database
Run_Install_Scripts_On_Server(Form_Data);
//albeit complex, most important part of script
Confirm_Tim_Is_Awesome();
}

Confirm_Tim_Is_Awesome() {
while (getTim().has_moustache()) {
echo "<em>You're still awesome. </em>";
}