title: PHP free memory after unset
body: <p>Its small code for test:</p>
  
  <pre><code>$strings = array('&lt;big string here (2 Mb)');
  $arr = array();
  
  //--&gt; memory usage here is 17.1Mb (checked by pmap)
  echo memory_get_usage();//0.5Mb 
  //(i know, that other 16.6Mb of memory used by process are php libraries)
  
  for($i = 0; $i &lt; 20; ++$i)
  {
      $strings_local = array_merge($strings, array($i));
      $arr[$i] = $strings_local;
      unset($strings_local);
  }
  
  //--&gt; memory usage here is 20.3Mb (checked by pmap)
  echo memory_get_usage();//3.7Mb
  //so, here its all ok, 17.1+3.2 = 20.3Mb
  
  for($i = 0; $i &lt; 20; ++$i)
  {
      unset($arr[$i]);
  }
  
  //--&gt; memory usage here is 20.3Mb (checked by pmap)
  //BUT?? i UNSET this variables...
  echo memory_get_usage();//0.5Mb
  </code></pre>
  
  <p>So, seems like php is not free memory, even if you <code>unset()</code> your variable. How can i free memory after unset?</p>