PHP printf 的有關問題

PHP printf 的問題
想把number of record 的內容print出來，可是在頁面上無法顯示，無錯誤提示資訊。
$link = new mysqli( 'localhost ', 'root ', ' ', 'book ');

if(mysqli_connect_errno())
{
echo ' Connection failed '.mysqli_connect_error();
}

$query = "insert into detail values
( ' ".$title. " ', ' ".$author. " ', ' ".$isbn. " ', ' ".$price. " ') ";
$result = $link-> query($query);
$num = $link-> affected_rows;

if ($result)
{
// echo $num. 'record inserted into database ';
//就是這一行想顯示出來，可是無法在頁面上顯示//
printf( "%c record stored in the database ",$num);
}

$link-> close();
}
請教到底怎麼回事？謝謝

------解決方案--------------------
c - the argument is treated as an integer, and presented as the character with that ASCII value.
d - the argument is treated as an integer, and presented as a (signed) decimal number.

printf( "%d record stored in the database ",$num);

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