使用PDO的php / mysql DateTime格式

I got problem by getting date value from mysql through php. After I added the date, then no thing returned. What's the error of the code?

The column datatype is DateTime in mysql.

After output as Json, I use the Json formatter online one, and it said it is not valid Json. why?

try {
    $conn = new PDO("mysql:host=host;dbname=db", "username", "pwd!");
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $Ticker = htmlspecialchars($_GET["Ticker"]);
    $Today = date('Ymd');

    $result = $conn->prepare("SELECT Ticker, date(dateOfRelease) as 'Release date', Amount FROM TradeQuote WHERE Ticker='$Ticker' GROUP BY Date(dateOfRelease)");

    $result = $result->fetchAll(); 
    $temp = array();

    foreach($result as $r) {

          $temp[] = array('Ticker' => (string) $r['Ticker'], 'Release Date' => (date) $r['Release date'], 'Price' => (string) $r['Price'], 'Amount' => (string) $r['Amount']); 

        }
    $table = $temp;
    $jsonTable = json_encode($table);
    } catch(PDOException $e) {
        echo 'ERROR: ' . $e->getMessage();
    }
    echo $jsonTable;

?>

'Release Date' => (date) $r['Release date'],

You're Typecasting to a PHP datatype of date..... there is no such datatype in PHP

If you're trying to format the date, then use PHP's standard date formatting functions and methods

EDIT

example

foreach($result as $r) {
    $releaseDate = new DateTime((string) $r['Release date']);
    $temp[] = array('Ticker' => (string) $r['Ticker'], 'Release Date' => $releaseDate->format('Y-m-d'), 'Price' => (string) $r['Price'], 'Amount' => (string) $r['Amount']); 

}