Permission on log file after cron

Hello every body.

Every morning at 7, a cron task is launched (with command php) and so create the log file for the day.
But this file's permission is -rw-r--r-- with user "bob" and group "www-data".
So the file is not writeable by www-data and the log file is not filled for the rest of the day.

The permission for logs directory is "drwxrwsr-x".

The umask of bob is 0007.

Os : debian jessie

How can change the default permission ?

In advance, thank you

PHP has its own umask: https://secure.php.net/umask