Memcache is a distributed object caching system and uses key-value for storing small data. Before you start calling Memcache code into PHP, you need to make sure that it is installed. That can be done using class_exists method in php. Once it is validated that the module is installed, you start with connecting to memcache server instance.

if (class_exists('Memcache')) {
  $cache = new Memcache();
  $cache->connect('localhost',11211);
}else {
  print "Not connected to cache server";
}

This will validate that Memcache php-drivers are installed and connect to memcache server instance running on localhost.

Memcache runs as a daemon and is called memcached

In the example above we only connected to a single instance, but you can also connect to multiple servers using

if (class_exists('Memcache')) {
  $cache = new Memcache();
  $cache->addServer('192.168.0.100',11211);
  $cache->addServer('192.168.0.101',11211);
}

Note that in this case unlike connect , there wont be any active connection until you try to store or fetch a value.

In caching there are three important operations that needs to be implemented

1. Store data : Add new data to memcached server
2. Get data : Fetch data from memcached server
3. Delete data : Delete already existing data from memcached server

Store data

$cache or memcached class object has a set method that takes in a key,value and time to save the value for (ttl).

$cache->set($key, $value, 0, $ttl);

Here $ttl or time to live is time in seconds that you want memcache to store the pair on server.

Get data

$cache or memcached class object has a get method that takes in a key and returns the corresponding value.

$value = $cache->get($key);

In case there is no value set for the key it will return null

Delete data

Sometimes you might have the need to delete some cache value.$cache or memcache instance has a delete method that can be used for the same.

$cache->delete($key);

Small scenario for caching

Let us assume a simple blog. It will be having multiple posts on landing page that get fetched from database with each page load. In order to reduce the sql queries we can use memcached to cache the posts. Here is a very small implementation

if (class_exists('Memcache')) {
  $cache = new Memcache();
  $cache->connect('localhost',11211);
    if(($data = $cache->get('posts')) != null) {
      // Cache hit
      // Render from cache
    } else {
      // Cache miss
      // Query database and save results to database
      // Assuming $posts is array of posts retrieved from database
      $cache->set('posts', $posts,0,$ttl);
    }
}else {
  die("Error while connecting to cache server");
}