Assigning const value to const error in php

<?php
 class Lala {
  const a = "a";
  const b = a . "b";
 }
?>

Parse error: syntax error, unexpected '(', expecting ',' or 开发者_开发知识库';' on line 4

What's the problem with it?

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From the documentation:

The value must be a constant expression, not (for example) a variable, a property, a result of a mathematical operation, or a function call.

See http://www.php.net/manual/en/language.oop5.constants.php

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mb

<?php
 class Lala {
  const A = "a";
  const B = self::A . "b";
 }
?>