Illegal offset type errors occur when you attempt to access an array index using an object or an array as the index key.
Example:
$x = new stdClass();
$arr = array();
echo $arr[$x];
//illegal offset type
Your $xml array contains an object or array at $xml->entry[$i]->source for some value of $i, and when you try to use that as an index key for $s, you get that warning. You'll have to make sure $xml contains what you want it to and that you're accessing it correctly.
I had a similar problem. As I got a Character from my XML child I had to convert it first to a String (or Integer, if you expect one). The following shows how I solved the problem.
foreach($xml->children() as $newInstr){
$iInstrument = new Instrument($newInstr['id'],$newInstr->Naam,$newInstr->Key);
$arrInstruments->offsetSet((String)$iInstrument->getID(), $iInstrument);
}