File system case sensitivity?

This is a question about file system case sensitivity

in file and directory names:

Will MacOS allow a file named 'file.txt' and a file

named 'File.txt' exist in the same directory

at the same time?

For that matter can a directory name 'aDirectory'

and an directory name 'adirectory' both exist in

the same directory at the same time?

I am confused because I am doing web related

dev work on localhost. I use php to validate the

existence of a file and use a name with a charater

in uppercase in the file name and php finds the file.

But, I use Fetch FTP client to upload a file with a

name the same as another file in the same location,

accept with a character set uppercase. It is successfully

placed according to the FTP client. It now looks like I

have both of the files in the same directory.

BUT, I delete one and both are removed.