why does this not work?

gistfile1.php

<?php

class Base {
  public static $value;
  
  public static function get() {
    return static::$value;
  }
}

class Child extends Base {
  public static $value = 'hello';
}

var_dump(Child::get() == 'hello');

Use static:: instead of self:: - see http://www.php.net/manual/en/language.oop5.late-static-bindings.php