May 30th, 2015
I tested many applications on PHP7 and only one of them had an issue: Magento 1.9. The issue was very easy to fix. I couldn't find a way to submit a patch because only Magento 2 was on GitHub, so here's the patch if you decide to upgrade.
Inside function getOutput(), change this line:
$out .= $this->getBlock($callback)->$callback();
$fname = $callback; $out .= $this->getBlock($callback)->$fname();
If you're interested to learn more about why this happens, read about the new Uniform Variable Syntax.
Update Nikita Popov suggested that you can even do this in 1 line like so: