Error message appears when using PHP5

I receive four copies of this error after I submit a post:

Object of class stdClass could not be converted to string in /.../sites/all/modules/relatedlinks/relatedlinks.module on line 1093.

The line on 1093 is

$set = array_intersect_assoc($set1, $set2) + array_diff_assoc($set1, $set2);

where $set1 and $set2 are arrays whose elements are objects of class stdClass. To prevent the error from appearing I used

$set = @array_intersect_assoc($set1, $set2) + @array_diff_assoc($set1, $set2);

I can't say I understand what the message means or whether it is a bug (or a fix) in PHP. The PHP version is 5.2.4 running on Linux. I don't see the error when using 5.2.5 on my Windows PC.