getThePrimaryKeyInYii2

1、You can get the last inserted ID like this:

Yii::app()->db->getLastInsertId();

2、If $model->id wouldn't work then use Yii::app()->db->getLastInsertId() or getPrimaryKey()

3、If your goal is to get the id that was assigned to the model that you just saved, 
  then after you do $model->save(), simply do $model->id to get it back. 
4、
you can also get the last inserted id of another model.

$std_id = Students::model()->findAll(array('order' => 'admission_no DESC','limit' => 1));

            foreach($std_id as $f) {

                echo  "Last Inserted Admission No:".$f['admission_no'];
                }
or

you can last inserted id in the  same model

Yii::app()->db->getLastInsertID();
    
5、$model->primaryKey or
$model->id // this is your primary key ite

6、In Yii2 last inserted id can be get using

Yii::$app->db->getLastInsertedID();
added for the people looking for the answer of same question in yii2