title: Why does the error "expected to be a reference, value given" appear?
body: <p>It fires out when I try to call function with argument by reference</p>
  
  <pre><code>function test(&amp;$a) ...
  </code></pre>
  
  <p>through</p>
  
  <pre><code>call_user_func('test', $b);
  </code></pre>