A useful technique I've found is to use interfaces for package- or application-wide constants, making it easy to incorporate them into any classes that need access to them:

<?php
interface AppConstants
{
   const FOOBAR = 'Hello, World.';
}

class Example implements AppConstants
{
   public function test()
   {
      echo self::FOOBAR;
   }
}

$obj = new Example();
$obj->test();  // outputs "Hello, world."
?>

I realize the same could be done simply by defining the constant in a class and accessing it via "class_name::const_name", but I find this a little nicer in that the class declaration makes it immediately obvious that you accessing values from the implemented interface.
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