title: what happened when i use multi ob_start() without ob_end_clean() or ob_end_flush()?
body: <p>i have reviewed php manual about the ob_start() ob_end_clean() ob_end_flush(). And i have seen  a different example about the subject, anyway i modified the example but i'm confused at this point. here is the script.</p>
  
  <pre><code>ob_start();
  echo "Hello x, ";
  
  ob_start();
  echo "Hello y, ";
  
  ob_start();
  echo "Hello z, ";
  
  ob_start();
  echo "Hello World";
  $ob_2 = ob_get_contents();
  ob_end_clean();
  
  echo "Galaxy";
  $ob_1 = ob_get_contents();
  ob_end_clean();
  
  echo " this is OB_1 : ".$ob_1;
  echo "&lt;br&gt;  and this is OB_2  : ".$ob_2;
  </code></pre>
  
  <p>And output of this script is:</p>
  
  <p><strong>Hello x, Hello y, this is OB_1 : Hello z, Galaxy</strong></p>
  
  <p><strong>and this is OB_2 : Hello World</strong></p>
  
  <p><strong>--------------------------------------------</strong></p>
  
  <p><em>Why the output isn't like that?</em></p>
  
  <p><strong>this is OB_1 : Hello x, Hello y, Hello z, Galaxy</strong></p>
  
  <p><strong>and this is OB_2 : Hello World</strong> </p>
  
  <p><em>And what is the point i have missed?</em></p>