Hi, @Chr
I’ll try to explain it as simple as possible, I hope I don’t oversimplify something:
In older PHP versions you could use the variable $HTTP_RAW_POST_DATA to access the data submitted to a website. The PHP developers realized that this function shouldn’t be used and with PHP 5.6 created a warning saying so. Unfortunately this warning also appears in software that isn’t using $HTTP_RAW_POST_DATA (like piwik) and is creating new errors. To solve this one can set always_populate_raw_post to -1 to disable $HTTP_RAW_POST_DATA and as a result also remove the warning.
In PHP 7.0 $HTTP_RAW_POST_DATA was removed permanently, so nothing needs to be done there.