(PHP 4 >= 4.3.0, PHP 5, PHP 7, PHP 8)
stream_set_write_buffer — Sets write file buffering on the given stream
$stream, int $size): int
   Sets the buffering for write operations on the given stream
   to size bytes.
  
streamThe file pointer.
size
       The number of bytes to buffer. If size
       is 0 then write operations are unbuffered.  This ensures that all writes
       with fwrite() are completed before other processes are
       allowed to write to that output stream.
      
Returns 0 on success, or another value if the request cannot be honored.
Example #1 stream_set_write_buffer() example
The following example demonstrates how to use stream_set_write_buffer() to create an unbuffered stream.
<?php
$fp = fopen($file, "w");
if ($fp) {
  if (stream_set_write_buffer($fp, 0) !== 0) {
      // changing the buffering failed
  }
  fwrite($fp, $output);
  fclose($fp);
}
?>