title: How to call the function by variable
body: <p>I hope this question is not too simple, but I have no idea how to do this</p>
  
  <pre><code>$book = 'book';
  $car = 'car';
  
  function $book()
  { 
   return "Hello, world!";
  }
  function $car()
  {
   return "WoW , The red car";
  }
  </code></pre>