title: Need more details on PHP pack function
body: <p>i have been trying to understand the pack function. I'm having trouble on this sort of code</p>
  
  <pre><code>pack( 'NNnNna*', $string);
  </code></pre>
  
  <p>what does <code>NNnNna*</code> mean? and also i saw others as <code>axa*</code> and <code>a4xa*</code>.</p>