code: public static function logBase($number = null, $base = 10)
      {
          $number = Functions::flattenSingleValue($number);
          $base = ($base === null) ? 10 : (float) Functions::flattenSingleValue($base);
  
          if ((!is_numeric($base)) || (!is_numeric($number))) {
              return Functions::VALUE();
          }
          if (($base <= 0) || ($number <= 0)) {
              return Functions::NAN();
          }
  
          return log($number, $base);
      }
docs: LOG_BASE.
  
  Returns the logarithm of a number to a specified base. The default base is 10.
  
  Excel Function:
  LOG(number[,base])
  
  @category Mathematical and Trigonometric Functions
  
  @param float $number The positive real number for which you want the logarithm
  @param float $base The base of the logarithm. If base is omitted, it is assumed to be 10.
  
  @return float