(PHP 4, PHP 5, PHP 7, PHP 8)
atan — Arc tangent
Returns the arc tangent of num
in radians.
atan() is the inverse function of
tan() , which means that
$num == tan(atan($num))
for every value of
num
that is in
the domain of atan().
num
The argument to process
The arc tangent of num
in radians.
Contrary to the current description, it should hold y == tan(atan(y)) for ALL y.
However, x == atan(tan(x)) only holds for those x which are in the range of atan, which are those x with -pi/2 < x < pi/2.
Of course, those equalities are limited by precision. On my machine
tan(atan(1000)) returns 1000.0000000001.
atan(tan(0)) returns 0 (correct).
atan(tan(M_PI)) returns -1.2246467991474E-16 instead of 0.