(PHP 4 >= 4.3.0, PHP 5, PHP 7, PHP 8)
stream_set_write_buffer — Sets write file buffering on the given stream
   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.
Приклад #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);
}
?>