enhancement: Prime number generator

[Viktorminator]

Hey @dbierer !
At first, thank you for the great book! I don't have your email, so github seems a right place to send you a message.
I've recently saw that algorithm for Prime number generator can be slightly improved (p.333 in the PHP 7 Programming Cookbook).
From

function generatePrimes($max)
{
    yield from [1, 2, 3];
    for ($x = 5; $x < $max; $x++) {
        if ($x & 1) {
            $prime = TRUE;
            for ($i = 3; $i < $x; $i++) {
                if (($x % $i) === 0) {
                    $prime = FALSE;
                    break;
                }
            }
            if ($prime) yield $x;
        }
    }
}

to

function generatePrimes($max)
{
    yield from [1, 2, 3];
    for ($x = 5; $x < $max; $x++) {
        if ($x & 1) {
            $prime = TRUE;
            for ($i = 3; $i <= sqrt($x); $i++) {
                if (($x % $i) === 0) {
                    $prime = FALSE;
                    break;
                }
            }
            if ($prime) yield $x;
        }
    }
}

Only slight change in for $i <= sqrt($x)
I hope you find it useful.

Best!