Thoughts on Lucene, Solr, Nutch and vertical search 

OK. The PHP manual explicitly states you CANNOT use an expression when assigning to a static variable.

You can, however, do this:

class MyClass {
  public static $a = 1;
  public static $b;

  public static function init() {
    self::$b = self::$a + 1;
  }
}
MyClass::init();

Nifty eh?