title: Reading Zend Engine API code: What does ## (double hash) mean?
body: <p>Out of curiousity, I'm reading the Zend Engine API code and encountered quite a number of ## in their #define's. For example, at /usr/lib/php5/Zend/zend_API.h:</p>
  
  <pre><code>#define ZEND_FN(name) zif_##name
  #define ZEND_MN(name) zim_##name
  </code></pre>
  
  <p>What does the ## (double hash) symbols mean in these two lines?</p>