The atan() function returns the arctangent of a number as a numeric value between -PI/2 and PI/2 radians.

<?php
echo(atan(0.50) . "<br />");
echo(atan(-0.50) . "<br />");
echo(atan(5) . "<br />");
echo(atan(10) . "<br />");
echo(atan(-5) . "<br />");
echo(atan(-10))
?>

ans:

0.463647609001
-0.463647609001
1.37340076695
1.4711276743
-1.37340076695
-1.4711276743