UPDATE: The provided snippet wasn't accurate enough, and actually there was no 
bug in it. What I tried to do was starting an event loop inside another triggered 
event, which failed. I managed to make it work by creating the new event inside a 
new fork with pcntl_fork(). I've read the libevent documentation and it says that 
a new base should be used, but when I call event_base_loop(), it blocks the 
previous one, and no new events are triggered in it until the new one ends. I'd 
like to know if there's any way to do it without pcntl_fork(). Am I missing 
something?

I've created a proof of concept here:

<?php

class Relay{

    private $connectionHandler;
    private $newHandler;
    
    public function test(){
        
        $this->connectionHandler = stream_socket_client("tcp://localhost:80", 
$errno, $errstr);

        if (!$this->connectionHandler) {
            echo "CRITICAL ERROR: $errstr ($errno)\n";
            exit;
        }

        stream_set_blocking($this->connectionHandler, 0);

        $this->eventBase = event_base_new();
        
        $buf = event_buffer_new($this->connectionHandler, array($this, 
'onReceive'), array($this, 'onWrite'), array($this, 'onError'), $this-
>eventBase);
        
        event_buffer_base_set($buf, $this->eventBase);
        event_buffer_priority_set($buf, 10);
        event_buffer_watermark_set($buf, EV_READ, 0, 0xFFFFFF);
        event_buffer_enable($buf, EV_READ | EV_PERSIST);
        
        event_base_loop($this->eventBase);
    }

    public function onReceive($buffer, $base) {

        $receiveBuffer = null;
        
        while ($rawData = event_buffer_read($buffer, 256)) {
            echo "RECEIVED DATA AT 1: ".$rawData;
            $receiveBuffer .= $rawData;
        }
        
        if (!$this->newHandler){
            
            echo "CREATING NEW CONNECTION...\n";

            $this->newHandler = stream_socket_client("tcp://localhost:999", 
$errno, $errstr);

            if (!$this->newHandler) {
                echo "CRITICAL ERROR: $errstr ($errno)\n";
                exit;
            }

            stream_set_blocking($this->newHandler, 0);

            if (-1 == ($pid = pcntl_fork())){
                echo "COULD NOT FORK";
                exit;
            }else if ($pid == 0){
                $this->createNewConnection($this->newHandler);
                exit;
            }else{ //parent process
		//do something
            }
            
        }
        
        fwrite($this->newHandler, $receiveBuffer);
        
    }
    
    public function onWrite($buffer, $args) {
        echo "WRITE AT 1\n";
    }
    
    public function onError($buffer, $base) {
        echo "ERROR AT 1\n";
    }
    
    public function onReceive2($buffer, $args){
        
        $receiveBuffer = null;
        
        while ($rawData = event_buffer_read($buffer, 256)) {
            echo "RECEIVED DATA AT 2: ".$rawData;
            $receiveBuffer .= $rawData;
        }
        
        fwrite($this->connectionHandler, $receiveBuffer);
        
    }
    
    public function onWrite2($buffer, $args) {
        echo "WRITE AT 2\n";
    }
    
    public function onError2($buffer, $args) {
        echo "ERROR AT 2\n";
    }
    
    private function createNewConnection($newHandler){
        
        $newBase = event_base_new();
        $buf = event_buffer_new($newHandler, array($this, 'onReceive2'), 
array($this, 'onWrite2'), array($this, 'onError2'), $newBase);
        event_buffer_base_set($buf, $newBase);
        event_buffer_priority_set($buf, 9);
        event_buffer_watermark_set($buf, EV_READ, 0, 0xFFFFFF);
        event_buffer_enable($buf, EV_READ | EV_PERSIST);
        event_base_loop($newBase);

    }
}

$relayTest = new Relay();
$relayTest->test();

I'd suggest you to take a look at some existing application using PECL/libevent.
phpDaemon https://github.com/kakserpom/phpdaemon/ is one of them.
If you still have some problems and think it's some PECL/libevent issue, please provide 
a short and self-sufficient reproduce case.

Regarding the documentation:
yes, the examples are missing and that is because nobody provides them.
Are you willing to help? I'd be more than happy to add your examples to the doc.