Under PLESK the file and directory owners are the domain owner's user name (owner) and psacln (group). FTP'd files get the same owner/group with rw-r--r--. There is no write right for the PHP scripts, which run as 'www-data'.
Options are to change the group to 'www-data', or allow write access for all (mode 777). The latter is often recommended. I doubt that this is the best solution, but did it for the moment as quick solution.
Update:
The above is only true if PHP runs as apache module, PLESK setting for PHP support is 'Apache module'.
If PHP runs under fast CGI, PHP is executed under the domain owner's user name. E.g. the PHP scripts have the same access rights as the owner.
No file access right changes necessary with FastCGI! PHP support should be set to 'FastCGI application'.