Creates a unique temporary filename in the specified directory. If the directory does not exist, tempnam() may generate a filename in the system's temporary directory.
The behaviour of the tempnam() function is system dependent. On Windows the TMP environment variable will override the dir parameter, on Linux the TMPDIR environment variable has precedence, while SVR4 will always use your dir parameter if the directory it points to exists. Consult your system documentation on the tempnam(3) function if in doubt.
Returns the new temporary filename, or the null string on failure.
See also tmpfile().