title: Get value of dynamically chosen class constant in PHP
body: <p>I would like to be able to do something like this:</p>
  
  <pre><code>class ThingIDs
  {
      const Something = 1;
      const AnotherThing = 2;
  }
  
  $thing = 'Something';
  $id = ThingIDs::$thing;
  </code></pre>
  
  <p>This doesn't work. Is there a straightforward way of doing something equivalent? Note that I'm stuck with the class; it's in a library I can't rewrite. I'm writing code that takes arguments on the command line, and I would <em>really</em> like it to take symbolic names instead of id numbers.</p>