Reading through the WordPress source, I found some syntax I didn't recognize.

if ( !function<em>exists('wp</em>set<em>current</em>user') ) :
function wp<em>set</em>current<em>user($id, $name = '') {
  global $current</em>user;
  if ( isset($current<em>user) &amp;&amp; ($id == $current</em>user->ID) )
    return $current<em>user;
  $current</em>user = new WP<em>User($id, $name);
  return $current</em>user;
}
endif;

A hunt revealed that this was an example of PHP's alternative syntax for control structures. You can replace an opening brace with a colon on some control structures, the closing brace with an end*;, where * is the control structure, and have blocks of multiple lines. Why would one use this? One example in the WordPress is a conditional function definition, which, okay, that's unusual, so maybe the unusual syntax makes it stand out, but in general I think using this is a terrible idea.