In SuPHP both the files and directories are owned by the user account and belongs to the user account’s group.
Therefore for Apache to read a file it is necessary to make the file readable by the anonymous nobody account. The same goes for directories (which also need to be executable). Therefore under SuPHP, normal files should be chmod 644 and directories should be chmod 755.
However, we don’t want the nobody user to be able to see our PHP files. This is because under SuPHP, PHP files are accessed using the user account’s privileges. Therefore PHP files can and should be chmod 600 (but 640 or 660 is acceptable, although not quite as secure).