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

<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->$rand_invoice_id=rand(0000,9999);$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);

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