setting up variable value in php 5.6.23

I am updating my sql query based on the selection made on checkbox

        $selected = ($data['selected'] == true) ? 1 : 0;
        echo $data['selected'];
        //prints true when checkbox is checked
        //prints false when unchecked
        echo $sql = "update test_table set selected = " . $selected . " where id = " . $data['product_id'];

But in both cases $selected is set to 1 . When

$data['selected'] == false it should save $selected = 0 but it does not.

What am I missing?