$chartDetails->allergies->$allergyId = $allergyTem this code run on localhost nice but on server gives error

Question

i working on patient chart table in which i have table charts (mysql) and using YII frameworm for developement. in chart table there are column id | chart_name | details | ....

in details column save value as array json_encode format. like allergydeatils

and many other information.

in array of allergies store multiple allergies identify with allergies id each.

sample code is here

$model = new PatientsAllergy;

$chart = MedicalCharts::model()->findByPk($chartid);

$chartDetails            = json_decode($chart->details);


$chartDetails->allergies->$allergyId = array(
                    'allergy_id' => $model->id,
                    'allergen' => Allergies::model()->findByAttributes(array('code' => $model->allergen_id))->name,
                    'allergy_location' => $model->allergy_location,
                    'allergy_reaction' => $model->allergy_reaction,
                    'allergy_severity_code' => $model->allergy_severity_code,
                    'allergy_start_date' => $model->allergy_start_date,
                    'is_active' => (($model->is_active == 1) ? ("Active") : ("InActive")),
                    'comments' => $model->comments
                );

in above code i store inforamation in $chartDetails->allergies->$allergyId and then it store in database

`$chart->details = json_encode($chartDetails);    
    $chart->save();

but in above code $chartDetails->allergies->$allergyId = array( /// data/////)

this line give me
error

`Creating default object from empty value     (path/protected/controllers/PatientsController.php:2761)`

on sever but on localhost it run nice no error.

please help.

thanking you !