Probably, $arg argument should be declared as mixed $arg = NULL, like it is in other methods. Its is useless to be just a string, since we want to pass more complex structures (array or object) and do so.(PECL event >= 1.2.6-beta)
EventBufferEvent::setCallbacks — Assigns read, write and event(status) callbacks
$readcb
   ,$writecb
   ,$eventcb
   ,$arg
    = ?Assigns read, write and event(status) callbacks.
readcb
    Read event callback. See About buffer event callbacks.
writecb
    Write event callback. See About buffer event callbacks.
eventcb
    Status-change event callback. See About buffer event callbacks.
arg
    A variable that will be passed to all the callbacks.
No value is returned.
