Imagewebp function doesn't exist in PHP 5.6?

PHP file:

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

      

        
        
        
      

    

Output:

5.6.0RC2
bool(false)

      

        
        
        
      

    

GD Support Included (GD Version: Included (2.1.0 Compatible))

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

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