code: public static function DELTA($a, $b = 0)
      {
          $a = Functions::flattenSingleValue($a);
          $b = Functions::flattenSingleValue($b);
  
          return (int) ($a == $b);
      }
docs: DELTA.
  
  Tests whether two values are equal. Returns 1 if number1 = number2; returns 0 otherwise.
  Use this function to filter a set of values. For example, by summing several DELTA
  functions you calculate the count of equal pairs. This function is also known as the
  Kronecker Delta function.
  
  Excel Function:
  DELTA(a[,b])
  
  @param float $a the first number
  @param float $b The second number. If omitted, b is assumed to be zero.
  
  @return int