是否有更好/更有效的方法来处理这个JSON数组？

I'm getting data from an API in JSON format. It's getting the resting heart rate of the user and inserting it into my database. It gets todays date as well as a couple days in the past and updates the database accordingly. The code below does work (although it's inserting a blank row for some reason). My question is, is there a better/more efficient way of writing this code?

Here is the array:

[activities-heart] => Array (
[0] => Array (
    [dateTime] => 2018-03-22
    [value] => Array (
        [customHeartRateZones] => Array ( )
        [heartRateZones] => Array (
            [0] => Array (
                [caloriesOut] => 1135.7736
                [max] => 85
                [min] => 30
                [minutes] => 814
                [name] => Out of Range
                )
            [1] => Array (
                [caloriesOut] => 1260.7179
                [max] => 119
                [min] => 85
                [minutes] => 289
                [name] => Fat Burn
                )
            [2] => Array (
                [caloriesOut] => 690.64515
                [max] => 145
                [min] => 119
                [minutes] => 90
                [name] => Cardio
                )
            [3] => Array (
                [caloriesOut] => 0
                [max] => 220
                [min] => 145
                [minutes] => 0
                [name] => Peak
                )
            )
        [restingHeartRate] => 65
        )
    )
[1] => Array (
    [dateTime] => 2018-03-23
    [value] => Array (
        [customHeartRateZones] => Array ( )
        [heartRateZones] => Array (
            [0] => Array (
                [caloriesOut] => 1512.00346
                [max] => 85
                [min] => 30
                [minutes] => 1113
                [name] => Out of Range
                )
            [1] => Array (
                [caloriesOut] => 1315.59604
                [max] => 119
                [min] => 85
                [minutes] => 280
                [name] => Fat Burn
                )
            [2] => Array (
                [caloriesOut] => 98.14618
                [max] => 145
                [min] => 119
                [minutes] => 13
                [name] => Cardio
                )
            [3] => Array (
                [caloriesOut] => 0
                [max] => 220
                [min] => 145
                [minutes] => 0
                [name] => Peak
                )
            )
        [restingHeartRate] => 64
        )
    )
)

And here is the relevant code to process it:

    <?php
    $obj = new RecursiveIteratorIterator( new RecursiveArrayIterator( json_decode( $return, TRUE ) ), RecursiveIteratorIterator::SELF_FIRST );

    foreach ( $obj as $key => $val ) {
        $$key = $val;

        $stmt = $connection->prepare( "INSERT INTO heartrate SET `encodedid` = ?, activitydate = ?, rhr = ? ON DUPLICATE KEY UPDATE rhr= ?" );
        $stmt->bind_param( 'ssii', $studentencodedid, $dateTime, $restingHeartRate, $restingHeartRate );
        $stmt->execute();

        if ( $stmt->affected_rows < 1 ) {
            header( "Location: " . $url . "/index.php?errormessage=Failed to updated the resting heart rate value." );
        exit;
        }

        $stmt->close();
    }

Thanks for any feedback.

Tim