Description:
------------
Can not export reflection method in the abstract class to the string value for method with default parameter, equals to 'self::class'

See https://3v4l.org/3TdIg, reproduced in all latest 5.6 versions.

Test script:
---------------
<?php

abstract class Foo
{
    private function bar($a = self::class) {}
}


$refMethod = new \ReflectionMethod('Foo', 'bar');
echo "Exported: ", $refMethod;

Expected result:
----------------
Exported: Method [ <user> private method bar ] {
  @@ /in/3TdIg 5 - 5

  - Parameters [1] {
    Parameter #0 [ <optional> $a = 'Foo' ]
  }
}

Actual result:
--------------
Exported: 
Process exited with code 139.