SQL语法错误使用'01：09：43 WHERE Book_id = 19 AND Date_issued = 2017-03-29 17:28:34'在第1行附近

Question

ERROR: Could not able to execute You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '06:09:43 WHERE Book_id = 19 AND Date_issued = 2017-03-29 17:28:34' at line 1

I've been getting the above error upon executing the following code -

if(isset($_GET['rt']))//rt is a variable I'm passing in an anchor tag
{
$dateis=$_GET['rt'];
}
if(isset($_GET['id']))//id is also sent along with rt
{
$bk_id=$_GET['id'];
}
$ret = 'returned successfully';
$date = date('Y-m-d H:i:s');
if(mysqli_query($conn, "UPDATE issued_books SET Date_returned = $date WHERE Book_id = $bk_id AND Date_issued = $dateis"))
{
    $fine = mysqli_query($conn, "SELECT DATEDIFF('$dateis', '$date') AS days");
    $fine = $fine*10;
    mysqli_query($conn, "UPDATE issued_books SET Fine_amt = (Fine_amt + $fine) WHERE Date_issued = $dateis");
    mysqli_query($conn, "UPDATE library_books SET Available = (Available + 1) WHERE Book_id = $bk_id");
    header("Location: return-books.php?book=".urlencode($ret));
} else{
    echo "ERROR: Could not able to execute " .mysqli_error($conn);
}

Answer 1

In your first update query use ' single quotes here

 Date_returned = '$date'

I also suggest to use pdo