You want to force all buffered data to be written to a filehandle.
fwrite($fh,'There are twelve pumpkins in my house.');
This ensures that There are twelve pumpkins in my house. is written to $fh.
To be more efficient, system I/O libraries generally don’t write something to a file when you tell them to.
Instead, they batch the writes together in a buffer and save all of them to disk at the same time.
Using fflush() forces anything pending in the write buffer to be actually written to disk.
Flushing output can be particularly helpful when generating an access or activity log.
Calling fflush() after each message to the logfile makes sure that any person or program monitoring the logfile sees the message as soon as possible.