PHP 5.5.x + offers webp support. However, this is not working with gentoo ebuild versions of PHP as the --with-vpx-dir is not set which enables webp support. Without this functionality, all the PHP webp GD functions such as http://php.net/manual/en/function.imagecreatefromwebp.php will not work. Thanks.
I'll look into adding this to do the next version bump
Latest php 5.5.16 should have webp support now. Please test.