(PECL libevent >= 0.0.4)
event_buffer_set_callback — Set or reset callbacks for a buffered event
$event
   , mixed $readcb
   , mixed $writecb
   , mixed $errorcb
   [, mixed $arg
  ] )
   Sets or changes existing callbacks for the buffered event.
  
event
     Valid buffered event resource.
readcb
     Callback to invoke where there is data to read, or NULL if no callback is desired.
writecb
     Callback to invoke where the descriptor is ready for writing, or NULL if no callback is desired.
errorcb
     Callback to invoke where there is an error on the descriptor, cannot be NULL.
arg
     An argument that will be passed to each of the callbacks (optional).
   Returns TRUE on success or FALSE on failure.