Sort array of associative arrays on multiple columns using specified sorting rules

In PHP 5.3 every parameter in the array has to be a reference when calling array_multisort() with call_user_func_array().

This function sorts a multidimensional array and shows a way to build an array of referenced params that works correctly.

function msort()
{
  $params = func_get_args();
  $array = array_pop($params);

  if (!is_array($array))
    return false;

  $multisort_params = array();
  foreach ($params as $i => $param) 
  {
    if (is_string($param)) 
    {
      ${"param_$i"} = array();
      foreach ($array as $index => $row) 
      {
        ${"param_$i"}[$index] = $row[$param];
      }
    }
    else 
      ${"param_$i"} = $params[$i];

    $multisort_params[] = &${"param_$i"};
  }
  $multisort_params[] = &$array; 

  call_user_func_array("array_multisort", $multisort_params);

  return $array;
}

Example:

$data is the given array from the question

$sorted_data = msort('city', SORT_ASC, SORT_STRING, 'zip', SORT_DESC, SORT_NUMERIC, $data)

This should work for the situation you describe.

usort($arrayToSort, "sortCustom");

function sortCustom($a, $b)
{
    $cityComp = strcmp($a['city'],$b['city']);
    if($cityComp == 0)
    {
        //Cities are equal.  Compare zips.
        $zipComp = strcmp($a['zip'],$b['zip']);
        if($zipComp == 0)
        {
            //Zips are equal.  Compare last names.
            return strcmp($a['last_name'],$b['last_name']);
        }
        else
        {
            //Zips are not equal.  Return the difference.
            return $zipComp;
        }
    }
    else
    {
        //Cities are not equal.  Return the difference.
        return $cityComp;
    }
}

You could condense it into one line like so:

function sortCustom($a, $b)
{
    return ($cityComp = strcmp($a['city'],$b['city']) ? $cityComp : ($zipComp = strcmp($a['zip'],$b['zip']) ? $zipComp : strcmp($a['last_name'],$b['last_name'])));
}

As far as having a customizable sort function, you're reinventing the wheel. Take a look at the array_multisort() function.