php associative array of objects

I'm trying to create associative array of objects from row result set with member id as the key, but getting some error.

addATravelog() is just a function of the class UserLogsAndSOS(), whose objects i want in array.

Here is what I tried:

class UserArraySet {

private $arrayOfUsers = array();    

function createArrayForTravelogs($result) {
    While($row = $result->next()) {
        if(array_key_exists($row['id'], $this->arrayOfUsers)) {
            $this->arrayOfUsers[$row['id']] = new UserLogsAndSOS();
        }
    $this->arrayOfUsers[$row['id']]->addATravelog($row['title'], $row['blog']); //line     72                                           
    }
}
}

On calling createArrayForTravelogs() from the object I got the following error Here is the error I got:

Notice: Undefined index: 1 in C:\xampp\htdocs\site\classes\userprofile.php on line 72

Fatal error: Call to a member function addATravelog() on a non-object in C:\xampp\htdocs\site\classes\userprofile.php on line 72

Can someone please let me know how to achieve this, I want something like this:

Array (

 [1] => objectUserLogsAndSOS1

 [5] => objectUserLogsAndSOS2
 ....
)

where key is the member id from $row. I also need to check if the key exists, then call a function of that particular object to add data to its member, if not then create an object and then call a function of that particular object to add data to its member.

Thanks