I faced the problem with wordpress installation in userdir. As the user as no permissions on that dir, PHP scripts can't write files to that dir. For example Worpdress can't create its configuration file on install, or install extensions.
This is bad....
My solution is to add 'my_user' to www-data group and to:
# sudo chmod -R 770 public html
# sudo chown -R my_user:www-data public html
This has to be done anytime 'my_user' adds or edits a php file in public_html. This is tedious and should be fixed as soon as possible.