函数imagewebp在PHP 5.6中不存在？

PHP-File:

<?php
echo phpversion();
echo "
";
var_dump(function_exists('imagewebp'));

Output:

5.6.0RC2
bool(false)

GD Support is enabled (GD Version:bundled (2.1.0 compatible))

---

Where is the problem? "imagewebp" should exist since PHP 5.5.0 (see: http://php.net/manual/en/function.imagewebp.php)

PS: (I'm not the only one... How to compile php to enable webp support?)