create file with name based on what already exist in folder

[dittu]

i want to create a new file in a folder with existing files with names in a numerical order eg. 1, 2, 3, 4...i want to check what is the last nr and then create a file with one nr over that one.i know i should use file_exists but i don't know exactly how to use it, in a for loop maybe? but how?would be nice if someone could give me a hint