(PECL memcache >= 0.2.0)
Memcache::increment — Increment item's value
$key
   [, int $value = 1
  ] )
   Memcache::increment() increments value of an item by
   the specified value. If item specified by
   key was not numeric and cannot be converted to a
   number, it will change its value to value.
   Memcache::increment() does not
   create an item if it doesn't already exist.
   
Also you can use memcache_increment() function.Note:
Do not use Memcache::increment() with items that have been stored compressed because subsequent calls to Memcache::get() will fail.
keyKey of the item to increment.
value
       Increment the item by value.
      
   Returns new items value on success  or FALSE on failure.
  
Example #1 Memcache::increment() example
<?php
/* procedural API */
$memcache_obj = memcache_connect('memcache_host', 11211);
/* increment counter by 2 */
$current_value = memcache_increment($memcache_obj, 'counter', 2);
/* OO API */
$memcache_obj = new Memcache;
$memcache_obj->connect('memcache_host', 11211);
/* increment counter by 3 */
$current_value = $memcache_obj->increment('counter', 3);
?>