echo "acos(0) = " . acos(0) . "<br />";
echo "acos(-1) = " . acos(-1) . "<br />";
echo "acos(1) = " . acos(1) . "<br />";
echo "acos(0.5) = " . acos(0.5) . "<br />";
/*
run:
acos(0) = 1.5707963267949
acos(-1) = 3.1415926535898
acos(1) = 0
acos(0.5) = 1.0471975511966
*/