You have numbers in degrees but want to use the trigonometric functions.
Use deg2rad() and rad2deg() on your input and output:
$degree = 90;
// cosine of 90 degrees is 0
$cosine = cos(deg2rad($degree));
By definition, 360 degrees is equal to 2π radians, so it’s easy to manually convert between the two formats.
However, these functions use PHP’s internal value of π, so you’re assured a high-precision answer.
To access this number for other calculations, use the constant M_PI, which is 3.14159265358979323846.
There is no built-in support for gradians. This is considered a feature, not a bug.