start(callable $callback = null)
Starts the process and returns after writing the input to STDIN.
This method blocks until all STDIN data is sent to the process then it returns while the process runs in the background.
The termination of the process can be awaited with wait().
The callback receives the type of output (out or err) and some bytes from the output in real-time while writing the standard input to the process. It allows to have feedback from the independent process during execution.
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