Find the side of a triangle using two sides and an angle:
<?
$a = "10"; // length of side a
$b = "15"; // length of side b
$angle = "45"; // angle opposite side c
$c = (sqrt(pow($a, 2) + pow($b, 2) - $a * $b * cos(deg2rad($angle)) * 2));
// side c = 10.613894770933
?>