<?php
  class Foo {
    const Bar = 'Foobar';
  
    function foobar($foobar = Foo::Bar) {
      echo $foobar;
    }
  }
  
  $o = new Foo;
  $o->foobar();
  ?>

works, but

  <?php
  class Foo {
    const Bar = 'Foobar';
  
    function foobar($foobar = self::Bar) {
      echo $foobar;
    }
  }
  
  $o = new Foo;
  $o->foobar();
  ?>

does not and prints "Fatal error: Undefined class 'self' in c:\test.php on line 5".