title: What does ${ } mean in PHP syntax?
body: <p>I have used PHP for a long time, but I just saw something like,</p>
  
  <pre><code>${  } 
  </code></pre>
  
  <p>To be precise, I saw this in a PHP Mongo page:</p>
  
  <pre><code>$m = new Mongo("mongodb://${username}:${password}@host");
  </code></pre>
  
  <p>So, what does <code>${ }</code> do? It is quite hard to search with Google or in the PHP documentation for characters like <code>$</code>, <code>{</code> and <code>}</code>.</p>