Returns the output incrementally.
In comparison with the getOutput method which always return the whole output, this one returns the new output since the last call.
string The process output since the last call
public function getIncrementalOutput() {
$data = $this
->getOutput();
$latest = substr($data, $this->incrementalOutputOffset);
$this->incrementalOutputOffset = strlen($data);
return $latest;
}