Description:
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It would be really useful to have the ability to create a tempdir, with a 
unique name, which we know we can write to, and will be automatically cleaned 
up.

For example, if I want to run an external command (with exec() ) which will 
write to a file, where:

  - it must be known that file-creation can succeed
  - the file does not exist at the moment


Test script:
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Some applications will refuse to overwrite an existing file (that we created with tempnam()), because they don't want to clobber it. What we effectively need is a race-free way to assign a temp-filename without actually creating the file.  Using tmpfile() doesn't help, because we can't pass a file-handle to php's exec().