error

$ php test.php 

Fatal error: Call to a member function get_string() on a non-object in /private/tmp/test.php on line 11

this.php

<?php 

class Foo {
  function get_string() {
    return "world";
  }

  function parse_string($string) {
    $y = $this;
    $callback = function ($match) use ($x, &$y) {
      return $x->get_string();
    };
    return preg_replace_callback("/{{name}}/", $callback, $string);
  }
}

$x = new Foo();

echo $x->parse_string("hello {{name}}") . "\n";