(PECL memcache >= 0.2.0)
Memcache::add — Add an item to the server
   Memcache::add() stores variable
   var with key only if such 
   key doesn't exist at the server yet.
   Also you can use memcache_add() function.
  
keyThe key that will be associated with the item.
varThe variable to store. Strings and integers are stored as is, other types are stored serialized.
flag
       Use MEMCACHE_COMPRESSED to store the item
       compressed (uses zlib).
      
expireExpiration time of the item. If it's equal to zero, the item will never expire. You can also use Unix timestamp or a number of seconds starting from current time, but in the latter case the number of seconds may not exceed 2592000 (30 days).
   Gibt bei Erfolg TRUE zurück. Im Fehlerfall wird FALSE zurückgegeben.
   Returns FALSE if such key already exist. For the rest
   Memcache::add() behaves similarly to
   Memcache::set().
  
Beispiel #1 Memcache::add() example
<?php
$memcache_obj = memcache_connect("localhost", 11211);
/* procedural API */
memcache_add($memcache_obj, 'var_key', 'test variable', false, 30);
/* OO API */
$memcache_obj->add('var_key', 'test variable', false, 30);
?>