(PECL memcache >= 0.2.0)
Memcache::add — 增加一个条目到缓存服务器
Memcache::add()方法在缓存服务器之前不存在key时，
以key作为key存储一个变量var到缓存服务器。
同样可以使用函数memcache_add()。
key将要分配给变量的key。
var将要被存储的变量。字符串和整型被以原文存储，其他类型序列化后存储。
flag
使用MEMCACHE_COMPRESSED标记对数据进行压缩(使用zlib)。
expire当前写入缓存的数据的失效时间。如果此值设置为0表明此数据永不过期。你可以设置一个UNIX时间戳或 以秒为单位的整数（从当前算起的时间差）来说明此数据的过期时间，但是在后一种设置方式中，不能超过 2592000秒（30天）。
成功时返回 true， 或者在失败时返回 false。
如果这个key已经存在返回false。 Memcache::add()方法的其他行为类似
Memcache::set()。
Example #1 Memcache::add()示例
<?php
$memcache_obj = memcache_connect("localhost", 11211);
/* 面向过程编程 API */
memcache_add($memcache_obj, 'var_key', 'test variable', false, 30);
/* 面向对象编程 API */
$memcache_obj->add('var_key', 'test variable', false, 30);
?>
It looks like add() function is truly 100% atomic, and safeadd bicycle mentioned in the other comment is useless. There are few links where developers of Memcahed explained it deeper
http://lists.danga.com/pipermail/memcached/2008-March/006647.html
http://www.serverphorums.com/read.php?9,214222
Race conditions happen on an heavy load server when more than one thread tries to execute memcache_add.
For example if thread A and thread B try to save the same key you can test that sometimes both return TRUE.
To have the right behaviour you can verify that the correct value is in the assigned key:
<?php
function memcache_safeadd(&$memcache_obj, $key, $value, $flag, $expire)
{
if (memcache_add($memcache_obj, $key, $value, $flag, $expire))
{
return ($value == memcache_get($memcache_obj, $key));
}
return FALSE;
}
?>
It's also good to note that add will succeed if the key exists but is expired
skeleton of a thread safe updater for an incremental counter:
<?php
$key = "counter";
$value = $memcache->increment($key, 1);
if ($value === false) {
// --- read from DB ---
$query = "SELECT value FROM database";
$result = mysql_query($query);
$row = mysql_fetch_assoc($result);
$db_value = $row["value"];
$add_value = $memcache->add($key, $db_value + 1, 0, 0);
if ($add_value === false) {
$value = $memcache->increment($key, 1)
if ($value === false) {
error_log ("counter update failed.");
}
} else {
$value = $db_value + 1;
}
}
// --- display counter value ---
echo $value;
?>
[c.2007]
if you read source code for MMC_SERIALIZED you will see at line ~1555 that [a line ~1560]
!(is_string,is_long,is_double,is_bool)
[is] serialized and that serialized values are flaged as MMC_SERIALIZED for return (fetch) code unserialize these values again