/{(.*?)}/

That means, match any character between { and }, but don't be greedy - match the shortest string which ends with } (the ? stops * being greedy). The parentheses let you extract the matched portion.

example:  preg_replace('/{(.*?)}/', '', $folder)

Another way would be

/{([^}]*)}/

This matches any character except a } char (another way of not being greedy)