What does yield mean in PHP?

I’ve recently stumbled over this code:

function xrange($min, $max) 
{
    for ($i = $min; $i <= $max; $i++) {
        yield $i;
    }
}

I’ve never seen this yield keyword before. Trying to run the code I get

Parse error: syntax error, unexpected T_VARIABLE on line x

So what is this yield keyword? Is it even valid PHP? And if it is, how do I use it?