title: php call_user_func variable error
body: <p>I'm just feeling my way around with some tests to get an idea of how to tackle a coding issue. Now, I'm getting behavior I don't understand.</p>
  
  <pre><code>function product($a, $b) {
  echo "Yo ".$a;
  echo "Yo ".$b;
  }
  
  call_user_func('product', 'dd', 'lizzy');
  </code></pre>
  
  <p>The above works, however,</p>
  
  <pre><code>$variablename = "'product','dd','lizzy'";
  call_user_func($variablename);
  </code></pre>
  
  <p>throws an error:</p>
  
  <blockquote>
    <p>Warning: call_user_func() expects parameter 1 to be a valid callback, function ''product','dd','lizzy'' not found or invalid function name</p>
  </blockquote>
  
  <p>Obviously $variablename contains the same values.</p>
  
  <p>Anyone any hint for me to put me back on track?</p>