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.