PHP -- usort is modifying contents of objects in array, how do I prevent this?

Question

I am using usort with a user comparison function to sort an array of objects. After running usort on an array of these objects, I've found that some of the values of the objects have changed along with their position in the array. What am I missing? I don't believe there are any side effects in my user comparison function. Does usort deconstruct/reconstruct objects some how?

Here is the the user comparison function I'm using:

private function SortArrayOfSegments($segments){
    foreach($segments as $segment){
        echo '<pre>';
        var_dump($segment);  
    }
    usort($segments, "AirObject::CompareSegments");
    foreach($segments as $segment){
        var_dump($segment);
        echo '</pre>';
     }
    return $segments;
}

public static function CompareSegments($a, $b){
    $interval = date_diff(date_create($a->StartDateTime->GetString()),
                date_create($b->StartDateTime->GetString()));
    if($interval->invert == 1){
        return 1;   
    }else if($interval->y == 0 && $interval->m == 0  && $interval->d == 0 
    && $interval->i == 0 && $interval->s == 0 && $interval->h == 0){
        return 0;   
    }else if($interval->invert == 0){
        return -1;  
    }  
}

The objects I'm using look like this :

object(AirSegment)#14 (12) {
      ["StartDateTime"]=>
      object(VDateTime)#27 (4) {
        ["date"]=>
        string(10) "2010-12-07"
        ["time"]=>
        string(8) "09:23:21"
        ["timezone"]=>
        string(0) ""
        ["utc_offset"]=>
        string(0) ""
      }
      ["EndDateTime"]=>
      object(VDateTime)#23 (4) {
        ["date"]=>
        string(10) "2010-12-07"
        ["time"]=>
        string(8) "13:23:21"
        ["timezone"]=>
        string(0) ""
        ["utc_offset"]=>
        string(0) ""
      }
      ["start_airport_code"]=>
      string(3) "SFO"
      ["start_city_name"]=>
      string(13) "San Francisco"
      ["end_airport_code"]=>
      string(3) "STL"
      ["end_city_name"]=>
      string(8) "St Louis"
      ["operating_airline"]=>
      string(15) "United Airlines"
      ["operating_flight_number"]=>
      string(3) "335"
      ["duration"]=>
      float(NAN)
      ["required_properties":protected]=>
      array(9) {
        ["StartDateTime"]=>
        bool(false)
        ["EndDateTime"]=>
        bool(false)
        ["start_airport_code"]=>
        bool(false)
        ["end_airport_code"]=>
        bool(false)
        ["operating_airline"]=>
        bool(false)
        ["operating_flight_number"]=>
        bool(false)
        ["start_city_name"]=>
        bool(false)
        ["end_city_name"]=>
        bool(false)
        ["service_class"]=>
        bool(true)
      }
      ["service_class"]=>
      string(5) "Coach"
      ["trip_id"]=>
      string(4) "1621"
    }

The property that is changing is the duration property. Before usort is run, every object has a valid float value. After usort, two of them are NaN.

RESOLUTION:

date_diff has side effects -- at least in my build of PHP. Removing it fixed the issues entirely.

public static function CompareSegments($a, $b){
    $adate = new DateTime($a->StartDateTime->GetString());
    $bdate = new DateTime($b->StartDateTime->GetString());
    $lessThan = $adate < $bdate;
    $equal = $adate == $bdate;
    $greaterThan = $adate > $bdate;

    if($lessThan){
        return -1;  
    }else if($equal){
        return 0;   
    }else{
        return 1;   
    }
}

Answer 1

Off the top of my head, I don't see anything in there that should be modifying the contents of the array elements themselves.

The way usort() works is that when it determines the new order of the elements, it adds new elements to the array which holds the value of the originals, then removes the originals, so strictly speaking, none of the elements of the array you passed into the function ever come out, only copies of them do (albeit as part of the same array variable you passed in).

I don't see any reason why usort would destruct and re-construct objects, as you asked, but it does do a similar thing to the array itself. As far as I can tell, nothing in the value of the array elements should be changed, unless you do it explicitly in the comparison function.

In your CompareSegments method, you make calls to a StartDateTime->GetString() method. Is it possible that the GetString() method is modifying your data?