php安装memcached并使用memcached，首先需要服务器有memcache服务运行，如果需要本地安装memcache服务，请参考这篇文章：
memcache安装教程
在安装php的memcached扩展前，我们需要先安装一个lib库，不然后面会报错：
php configure: error: memcached support requires libmemcached.
前往libmemcache下载：
https://launchpad.net/libmemcached/+download
安装如下：
root [~]# tar -zxvf libmemcached-1.0.18.tar.gz
root [~]# cd libmemcached-1.0.18
root [~]# ./configure --with-memcached
root [~]# make
root [~]# make install
安装完成后，我们开始安装php memcached。
前往php扩展网站下载memcached：
http://pecl.php.net/package/memcached
安装如下：
root [~]# tar -zxvf memcached-2.2.0.tgz
root [~]# cd memcached-2.2.0
#你的php路径
root [~]# /php/bin/phpize
root [~]# ./configure --with-php-config=/opt/app/php/bin/php-config
root [~]# make
root [~]# make install
安装完成后，会告知我们动态链接库在哪里：
Installing shared extensions: /***/php/lib/php/extensions/no-debug-non-zts-20131226/
建议复制到extensions目录下，并重新配置php：
root [~]# cp no-debug-non-zts-20131226/memcached.so ./
root [~]# cd /***/php/
root [~]# vi php.ini
#加入一行声明
extension=memcached.so
重启php-fpm：
此时，可以看到php安装memcached完成：
memcached
memcached support => enabled
libmemcached version => 1.0.18
memcached.compression_factor => 1.3 => 1.3
memcached.compression_threshold => 2000 => 2000
memcached.compression_type => fastlz => fastlz
memcached.serializer => php => php
memcached.sess_binary => 0 => 0
memcached.sess_connect_timeout => 1000 => 1000
memcached.sess_consistent_hash => 0 => 0
memcached.sess_lock_expire => 0 => 0
memcached.sess_lock_max_wait => 0 => 0
memcached.sess_lock_wait => 150000 => 150000
memcached.sess_locking => 1 => 1
memcached.sess_number_of_replicas => 0 => 0
memcached.sess_prefix => memc.sess.key. => memc.sess.key.
memcached.sess_randomize_replica_read => 0 => 0
memcached.sess_remove_failed => 0 => 0
memcached.sess_sasl_password => no value => no value
memcached.sess_sasl_username => no value => no value
memcached.store_retry_count => 2 => 2
memcached.use_sasl => 0 => 0
Registered save handlers => files user memcached
php安装memcache和memcached类似，需要去memcache下载源码编译即可：
http://pecl.php.net/package/memcache
php session默认使用file，存储在/tmp下面，为了减少磁盘IO和分布式缓存，我们可以将php的session设置为memcache或memcached，二者在配置php.ini时有些差异。
设置memcache：
session.save_handler = memcache
session.save_path = "tcp://localhost:11211"
设置memcached：
session.save_handler = memcached
session.save_path = "localhost:11211"
设置memcached时，无法直接通过php读取到memcached内容，而通过设置memcache可以直接读取内容。
测试代码如下：
session_start();
if (!isset($_SESSION['TEST'])) {
$_SESSION['TEST'] = time();
}
$_SESSION['TEST2'] = time();
print $_SESSION['TEST'];
print "<br><br>";
print $_SESSION['TEST2'];
print "<br><br>";
print session_id();
print "<br><br>";
$memcache = new Memcached;
$memcache->addServer('localhost', 11211);
var_dump($memcache->get(session_id()));
print "<br><br>";
$memcache->set('aaaa', 'hello ranshy');
print $memcache->get('aaaa');
显示结果：
1484898619
1484898899
1jldgkbr59hi41be52mdpumig1
string(37) "TEST|i:1484898619;TEST2|i:1484898898;"
hello ranshy
ranshy
test