test-static-cache.php

<?php

class A {
  public $bar;

  public function foo() {
    static $cache = [];
    if (!array_key_exists($this->bar, $cache)) {
      $cache[$this->bar] = mt_rand();
    }
    var_dump($cache);
    return $cache[$this->bar];
  }

}

(function(){
  var_dump('a');
  $a = new A();
  $a->bar = 'aaa';
  $a_foo = $a->foo();
  var_dump($a_foo);

  var_dump('b');
  $b = new A();
  $b->bar = 'bbb';
  $b_foo = $b->foo();
  var_dump($b_foo);

})();