Do not lose your time to install it on Ubuntu just trying "sudo apt-get install php5-memcached". There is something you need to do that sure installing memcached. Anyway...

Step 1.
$ sudo apt-get install memcached
Step 2.
$ sudo apt-get install php5-memcached
Step 3.
$ sudo /etc/init.d/apache2 restart

Ready!

What about some test?

<?php
error_reporting(E_ALL & ~E_NOTICE);

$mc = new Memcached();
$mc->addServer("localhost", 11211);

$mc->set("foo", "Hello!");
$mc->set("bar", "Memcached...");

$arr = array(
    $mc->get("foo"),
    $mc->get("bar")
);
var_dump($arr);
?>

Hoping to help someone.
~Kerem

For PHP 7 seems to work:

    $ sudo apt-get install memcached
    $ sudo apt-get install php-memcached
    $ sudo apachectl graceful

## php 7.2.19 / ubuntu 18.04.1

$mc = new Memcached();
$mc->addServer("localhost", 11211);
...

i'm planning on using membase for my website and i was really struggling to install the memcashed php client on ubuntu with pear, but actually there is an easier way:

$ sudo apt-get install php5-memcached

you dont even need to install the membase server first - this can be done afterwards. hope this helps someone!

If your server has --enable-json=shared as mine did, this extension requires you to have extension=json.so to load properly.