title: PHP First day of month using DateTime
body: <p>I have a datetime in php:</p>
  
  <pre><code>$datetime=new DateTime();
  </code></pre>
  
  <p>How can I get day of week:</p>
  
  <pre><code>$datetime-&gt;format("w");
  </code></pre>
  
  <p>But for first day of current <code>$datetime</code> month?</p>