Question

function find_highest_prime_factor($n)
{
    for ($i = 2; $i <= $n; $i++) 
    {   
        if (bcmod($n, $i) == 0) //its a factor
        {
            return max($i, find_highest_prime_factor(bcdiv($n,$i)));
        }
    }
    if ($i == $n)
    {
        return $n; //it's prime if it made it through that loop
    }
}

UPDATE: This is the correct answer, my bad!

Answer 1

Get rid of the final if statement otherwise if $i!=sqrt($n) because sqrt($n) is not an integer you have an undefined return value

function find_highest_prime_factor($n){
  for ($i = 2; $i <= sqrt($n); $i++) //sqrt(n) is the upperbound
  {
       if (bcmod($n, $i) == 0) //its a factor
       {
          return max($i, find_highest_prime_factor(bcdiv($n,$i)));
       }
   }
   return $n; //it's prime if it made it through that loop
 }