title: sudo mkdir in /usr/, Operation not permitted. El Capitan
body: <p>Attempting to install PHP, which requires the creation of an extensions directory within <code>/usr/lib/php/extensions</code>. The installation returned an error "Operation not permitted".</p>
  
  <p>I have since found out, <code>sudo</code> is not able to create any directories under <code>/usr/</code>. Anyone else experienced this?</p>