In looking to test his fputscsv functionality, David Otton found a simple way to measure its performance by using streams.
Then I realised I could use PHP's (fairly) new IO streams to dump the function's output to a temporary buffer, and read it back in for comparison. Not perfect, but it removes concerns about file mutexes, permissions, unique filenames, etc. and speeds up the tests, as they never touch disc.
He uses a custom stream and points it to php://memory to store and read the data from. Code is included in the post as well as example usage. It runs an assert that the value pushed into another memory chunk is the same as the first one (ensuring that the results of his fputcsv calls are valid).