Use php://input instead of raw_post_data to support PHP 5.6.

php_stream handling largely taken from PECL HTTP:

http://git.php.net/?p=pecl/http/pecl_http.git;a=blob;f=php_http_env.c;h=30ee32d7c68b3341aeaeb24c909b102537caccdf;hb=8ec2c825719482e62222163a300b0e18319591d0#l229

Copyright (c) 2004-2014, Michael Wallner <mike@iworks.at>.
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Bug-Debian: https://bugs.debian.org/745600