title: What are the technical differences between the Thread Safe and Non Thread safe PHP Windows Installation Packages?
body: <p>I'm currently about to install PHP for an Apache/Windows-based development environment, but it seems I'm about to fall at the first hurdle: Choosing the right package to install.</p>
  
  <p><a href="http://windows.php.net/download/" rel="noreferrer">PHP is available in no less than <em>four</em> flavours</a>:</p>
  
  <ul>
  <li>VC9 x86 Non Thread Safe </li>
  <li>VC9 x86 Thread Safe </li>
  <li>VC6 x86 Non Thread Safe</li>
  <li>VC6 x86 Thread Safe</li>
  </ul>
  
  <p>What's the difference between these versions in a <em>practical</em> sense?</p>
  
  <p>If this wasn't complicated enough, version 5.3 of PHP is only available in VC9 (with 5.2 coming with the VC6 packages). And yet, according to the PHP site, you should <strong>not</strong> use VC9 with Apache... So why does Apache get the older version?</p>
  
  <p>It's all very confusing and I'd like some help understanding the choices.</p>