title: NULL in WHERE clause
body: <p>I have the following prepared statement.  All is good...</p>
  
  <pre><code>$stmt=$conn-&gt;prepare('UPDATE t1 SET c1=? WHERE c2=? AND c3!=?');
  $stmt-&gt;execute(array(1,2,3));
  </code></pre>
  
  <p>But now I wish to use the same prepared statement, but include NULL values in the WHERE clause.</p>
  
  <pre><code>$stmt-&gt;execute(array(1,NULL,NULL));
  </code></pre>
  
  <p>Will this resolve to <code>UPDATE t1 SET c1=1 WHERE c2=NULL AND c3!=NULL</code>?  Isn't the proper query <code>UPDATE t1 SET c1=1 WHERE c2 IS NULL AND c3 IS NOT NULL</code>?  How can I have a single prepared statement which allows both NULL and not NULL values in the WHERE clause?</p>