这是为什么 Fatal Error,该怎么解决

这是为什么 Fatal Error
我只要一加入以下代码 网页加载就很慢 然后就会出现这个错误提示： Fatal error: Maximum execution time of 30 seconds exceeded in E:\xampp\htdocs\business\lib\functions.php on line 13

PHP code

$rand_invoice_id=rand(0000,9999);
$current_time=date("Y-m-d H:i:s");
$current_time_start=date("Y-m-d")." "."07:00:00";
$tomorrow_time_start=date("Y-m-d", strtotime('+1 day'))." "."07:00:00";
$yesterday_time_start=date("Y-m-d", strtotime('-1 day'))." "."07:00:00";
$row=1;
do
{
    $query="SELECT * FROM CART WHERE (invoice_id='$rand_invoice_id') AND (order_time BETWEEN '$current_time_start' AND '$tomorrow_time_start')";
                    
    $result=queryMysql($query);
    $rows=mysql_num_rows($result);
    if($rows != 0)
    {
        $rand_invoice_id=rand(0000,9999);
    }

}
while($row!=0);