title: Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given
body: <p>I get this error whenever I run this:</p>
  
  <pre><code>Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given
  </code></pre>
  
  <p>The Code:</p>
  
  <pre><code>$amn = mysql_query("SELECT * FROM `Messages` WHERE to_user='$usr' AND read='0'");
  print_r(mysql_num_rows($amn));
  </code></pre>