$capacity is count of stack in buffer.for example :<?phpuse parallel\{Channel,Runtime};$test = function($ch){ var_dump($ch->Recv()); var_dump($ch->Recv()); var_dump($ch->Recv()); echo 'Sleep'.PHP_EOL; sleep(3); echo 'Call Recv()'.PHP_EOL; var_dump($ch->Recv());};$t1 = new Runtime();$ch = new Channel(3); //buffered channel $ch->Send('abc1');$ch->Send('abc2');$ch->Send('abc3');$t1->Run($test,[$ch]);sleep(1);echo 'Wait for Recv()...'.PHP_EOL;$ch->Send('abc4'); #php waits for only one stack of channel to be empty(by Recv() method)?>result:string(4) "abc1"string(4) "abc2"string(4) "abc3"SleepWait for Recv()...Call Recv()string(4) "abc4"