Because a single colon is used in an alternative if/else/foreach etc syntax:

  <?php if($var == true): ?>  
  Hello world  
  <?php else: ?>  
  Cake is a lie  
  <?php endif; ?>

Edit: syntax

It's also used in the ternary operator

  x = condition ? if_true : if_false;

stun taken - I seriously had forgotten about that.