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Find all Pythagorean triangles with length or height less than or equal to 20

Pythagorean triangles are right angle triangles whose sides comply with the following equation:

a * a + b * b = c * c

where c represents the length of the hypotenuse, and a and b represent the lengths of the other two sides. Find all such triangles where a, b and c are non-zero integers with a and b less than or equal to 20. Sort your results by the size of the hypotenuse. The expected answer is:

[3, 4, 5]
[6, 8, 10]
[5, 12, 13]
[9, 12, 15]
[8, 15, 17]
[12, 16, 20]
[15, 20, 25]
php
for ($x = 1; $x <= 20; $x++) {
for ($y = $x; $y <= 20; $y++) {
$z = hypot($x, $y); // or $z = sqrt($x*$x + $y*$y);
if (round($z) == $z) {
$array = array($x, $y, $z);
sort($array, SORT_NUMERIC);
$res[] = $array;
}
}
}

// calculate the total of the sides
foreach ($res as $a) {
$total[] = ($a[0] + $a[1] + $a[2]);
}
array_multisort($total, $res); // and sort them after the total
// result is in $res

Greatest Common Divisor

Find the largest positive integer that divides two given numbers without a remainder. For example, the GCD of 8 and 12 is 4.