$capacity is count of stack in buffer.

for example :

<?php
use 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"
Sleep
Wait for Recv()...
Call Recv()
string(4) "abc4"

Please note $capacity unit is 1 MiB each, not bytes