Help with decimal to hexidecimal program

Well, I want to thank the people that helped me with my previous number conversion programs, but now I have another that I have some problems with. I've been working on this decimal to hexidecimal program. It outputs strange characters. However, it gives the correct answer with numbers from 0 to 15. Any help would be greatly appreciated.

Code:

---

#include <iostream.h>
#include <math.h>

//Converts the decimal to a hexidecimal number which appears as a string
//Uses the math.h header file
void dectohex (unsigned long int num, char *phex)
{
        char hexnumbers[] = {'0', '1', '2', '3', '4', '5', '6', '7',
                                                          '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'};
  int count = 0;

  for(int x = 7; x > -1; x--)
  {
          if (pow(16, x) < num)
      {
              for(int y = 0; y < 16; y++)
        {
                if (pow(16, x) * y > num)
            {
                    phex[count] = hexnumbers[y];
              num = num - pow(16, x) * y;
              count++;
            }
            else if (pow(16, x) * y == num)
            {
                    phex[count] = hexnumbers[y];
              phex[count + 1] = '\0';
              return;
            }
        }
      }
      else if (pow(16, x) == num)
      {
        phex[count] = hexnumbers[1];

              for(int c = count; c >= x; c++)
        {
            phex[c] = hexnumbers[0];
        }

        phex[count + x + 1] = '\0';
        return;
      }
  }
}

//Main function

int main(void)
{
        char hexstring[7];
  unsigned long int number;

  cout << "Enter a number: ";
  cin >> number;

  dectohex(number, hexstring);
  cout << "The hexadecimal number is: " << hexstring;

  return 0;
}

---

Thank you

Cstyle:
printf("%x", number);

C++style
cout<<hex<<number;

I believe that's what the commands are, but you'll want to try them and see.