[php-list] file creator

[cosminx2003]

I want to build a file creator.
Look how it will work : it will create a file file.txt, will rename it
in .zip|.rar|.iso|etc and then will put a filesize (unlimited
filesize), but i don't know how to put a specific filesize to a file,
is it possible? For example "file.txt" has 0 bytes, i want to change
its filesize to 100 bytes. Is it possible? if yes with what function?