title: What characters can immediately follow a parsed PHP variable using simple syntax?
body: <p>For instance, the following will not print <code>"hello john_doe"</code> as desired, since <code>$first_</code> is not defined.</p>
  
  <pre><code>$first = 'john';
  $last = 'doe';
  
  echo "hello $first_$last";
  </code></pre>
  
  <p>Maybe there are none?</p>