Server One variable multiple users can be modified at the same time, how to implement

Server One variable multiple users can be modified at the same time, how to implement?
For example: I set the variable $ A,
Multiple members, called $ A, are the same object, not separate. But it cannot be implemented with a database.
$a = 100;

When Zhang San, visit $ A and change $ A to 5;
At this point, John Doe access $ A, see $ A $5, John Doe change $ A to 50,

Multiple users can modify $ A and more can be seen. Similar to shared variables.

How is it implemented?

------Solution--------------------
Public variable class PHP code based on Memcache

Class Tshare {  private static $_instance;  Private $memcache;  Private Function __construct () {    $this->memcache = new Memcache;    $this->memcache->connect (' 127.0.0.1 ', 11211) or Die ("Could not Connect");  }  public static function getinstance () {    if (empty (self::$_instance)) self::$_instance = new self ();    return self::$_instance;  }  function __set ($name, $val) {    $this->memcache->set ($name, $val, false, +);  }  function __get ($name) {    return $this->memcache->get ($name);  
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