Question

I’ve been studying WordPress source code that operates on the filesystem, when I hit these few lines and I’m really not quite sure what they do?

$stat = stat( dirname( $new_file ));
$perms = $stat['mode'] & 0000666;
@ chmod( $new_file, $perms );

Answer 1

That code uses bitwise operations to ensure that a file’s permissions are no higher than 666.
To break it down:

// Retrieves the file details, including current file permissions.
$stat = stat( dirname( $new_file )); 

// The file permissions are and-ed with the octal value 0000666 to make
// sure that the file mode is no higher than 666. In other words, it locks
// the file down, making sure that current permissions are no higher than 666,
// or owner, group and world read/write.
$perms = $stat['mode'] & 0000666; 

// Finally, the new permissions are set back on the file
@chmod( $new_file, $perms );