php does not work with ubuntu 12.04.2 libevent-2.0-5

Setup
installed a new ubuntu 12.0.4.2 and setup as following:

sudo -s
wget O http://packages.couchbase.com/ubuntu/couchbase.key | apt-key add -
wget -O /etc/apt/sources.list.d/couchbase.list http://packages.couchbase.com/ubuntu/couchbase-ubuntu1204.list
apt-get -q -y update
apt-get -q -y install vim
apt-get -q -y install curl
apt-get -q -y install build-essential
apt-get -q -y install acl
apt-get -q -y install libssl0.9.8
apt-get -q -y install php5
apt-get -q -y install unzip
apt-get -q -y install libevent-2.0-5
apt-get -q -y install php-pear
apt-get -q -y install libcouchbase2-core libcouchbase2-bin libcouchbase-dev

Run the following script:
#!/usr/bin/php
<?php
$cb = new Couchbase("192.168.56.101", "Tommy", "", "Tommy");
$result = $cb->add("test", 1);
?>

Error message:
PHP Fatal error: Uncaught exception 'CouchbaseLibcouchbaseException' with message 'Failed to create libcouchbase instance' in /home/patrick/add.php:3
Stack trace:
#0 /home/patrick/add.php(3): Couchbase->__construct('192.168.56.101', 'Tommy', '', 'Tommy')
#1

thrown in /home/patrick/add.php on line 3

Work around:
Once libcouchbase-libevent is installed, it works.

sudo apt-get install libcouchbase-libevent

The ruby sdk works no matter if libcouchbase-libevent is installed or not.

Expectation
The php sdk to be able to use any backend.