Cannot use assign-op operators with overloaded objects nor string offsets

 Error Details: 500 - Cannot use assign-op operators with overloaded objects nor string offsets in /abc/abc/bbc/bbbc/cccccc/dddd/fffff/ggg.php on line 34. Error Type: 1

Why error is come:- 

$data['class'] .= ($row->required == 1) ? ' required' : '';

solution:- 

replace the line like below

$data['class'] = $data['class'].($row->required == 1) ? ' required' : '';

after the replace the line the error is not come try and comment.