title: PHP:: three echo statements with the same result
body: <p>Perhaps this is a petty question, but consider the following PHP code:</p>
  
  <pre><code>$a = "foo1"; $b = "foo2"; $c = "foo3";
  
  echo $a, $b, $c;
  
  echo $a . $b . $c;
  
  echo "$a$b$c";
  </code></pre>
  
  <p>aren't these three statements equivalent. What's the difference.
  What if one cannot decide whether to use one or the other?</p>