This function returns the value for the process PHP is executing under.
/var/somebody/files was only executable by owner (700).
It contained world-readable files, but without execute permission, PHP could not access the directory to download the files.
is_executable() returned false.
Making the dir world-executable (701) gave access to PHP. The download worked. And is_executable() returned true.
I assume that if I added PHP to the group of the folder and gave the group executable permission, that would also work.
