Error: Fatal error: Cannot redeclare exp() in PHP

Error: Fatal error: Cannot redeclare exp() in 

The message says Cannot redeclare and that means that the function already exists.

exp() is a built-in function in php.

PHP exp() Function 2 

You can do it like this

function my_exp($value1,$value2) {
  return $value1 ** $value2;
}
echo my_exp(2,8);//prints 256;

Or you can use the built-in function pow()

echo pow(2,8);//prints 256