how to store the primarkey values to the foreign key at same time of save

i am new to the laravel and trying to store the primarykey values of patientid to the foreign key of patient_id in address table

$patdet = $request->patdet; foreach($patdet as $patdets) {

    $pdet = new Patient();

    $pdet->fname = $patdets['fname'];
    $pdet->mname =  $patdets['mname'];
    $pdet->lname =  $patdets['lname'];
    $pdet->age   =  $patdets['age'];
    $pdet->blood_group = $patdets['bloodgroup'];
    $pdet->gender  = $patdets['gender'];
    $ptid = $pdet->save();
   }

$addr = $request->address[0];

                $address=new Address;

                $address->gps_lat = $addr['gps_lat'];
                $address->gps_log = $addr['gps_long'];
                $address->house_no  = $addr['houseno'];
                $address->zipcode    =  $addr['zip_code'];
                $address->street    = $addr['street'];
                $address->chowk  = $addr['chowk'];
                $address->city   = $addr['city'];
                $address->patient_id = $patid->id;
                $address->save();

i know that $ptid is local variable and cannot be used in address table ? so how can i store it in address table ??

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