title: Adding spaces at specific positions in a numeric string
body: <p>I would like to add spaces between set of numbers with <code>preg_replace()</code>;</p>
  <p><code>$numbers = '90000800123987';</code></p>
  <p>Desired output: <code>900 00 800 123 987</code></p>
  <p>I'm quite confused at this point, I couldn't get the desired output so far.</p>