Syntax of acos function
float acos(float value)
Returns the arc cosine of value in radians
Example of acos function in PHP
<?php
echo(acos(0.64) . "<br>");
echo(acos(-0.4) . "<br>");
echo(acos(0) . "<br>");
echo(acos(-1) . "<br>");
echo(acos(1) . "<br>");
echo(acos(2));
?>
Output
0.87629806116834
1.9823131728624
1.5707963267949
3.1415926535898
0
NAN