title: Execute default PHP function or custom function as param in function
body: <p>I wrote a simple function that accepts two params: a string and a function.</p>
  
  <p>I want that function return a value if the function passed from user with the given string is true, but how can I do this with PHP default functions passed as param?</p>
  
  <p>For example, if I use</p>
  
  <pre><code>call_user_func( 'empty', 'test' );
  </code></pre>
  
  <p>It doesn't works, but with custom functions yes (obviously).</p>
  
  <p>How can I do this?</p>
  
  <p>Thanks.</p>