title: MySQLi prepared statement complains that "only variables should be passed by reference"
body: <p>Code:</p>
  
  <pre><code>$stmt-&gt;bind_param("s", md5($input['user'] . $config['salt']));
  </code></pre>
  
  <p>PHP Error Message:</p>
  
  <blockquote>
    <p>Only variables should be passed by reference</p>
  </blockquote>
  
  <p>I've been working on this project but I am stuck now. I am new to PHP. What to do?</p>