title: what does a php function return by default?
body: <p>If I return nothing explicitly, what does a php function exactly return?</p>
  
  <pre><code>function foo() {}
  </code></pre>
  
  <ol>
  <li><p>What type is it?</p></li>
  <li><p>What value is it?</p></li>
  <li><p>How do I test for it exactly with === ?</p></li>
  <li><p>Did this change from php4 to php5?</p></li>
  <li><p>Is there a difference between <code>function foo() {}</code> and <code>function foo() { return; }</code></p></li>
  </ol>
  
  <p>(I am not asking how to test it like <code>if (foo() !=0) ...</code>)</p>