acos

Parameter:
$num: A float value between -1 and 1. (Outside this range will return NAN)

Returns:
The arc cosine of the number in radians.

NAN (Not A Number) if the input is outside [-1, 1].

<?php
$cosValue = 0.5;
$angle = acos($cosValue); // returns angle in radians
echo $angle; // 1.0471975512 (which is ~60 degrees)

echo rad2deg($angle); // Convert to degrees: ~60
?>