csv導出，支持大批量數據

<?php
    header ( "Content-type:application/vnd.ms-excel" );
    header ( "Content-Disposition:filename=" . iconv ( "UTF-8", "GB18030", "mycsv" ) . ".csv" );

    // 打開PHP文件句柄，php://output 表示直接輸出到浏覽器
    $fp = fopen('php://output', 'a'); 
    $sql = "select count(*) as num from ".$tab;
    $result = doSqlCyan($sql);
    $total_export_count = $result["num"];
    $pre_count = 2000;
    for ($i=0;$i<intval($total_export_count/$pre_count)+1;$i++){
        $sql = "select * from ".$tab." order by id asc limit ".strval($i*$pre_count).",{$pre_count}";
        DB::query("set names 'utf8'");
        $export_data = DB::fetch_all($sql);
        foreach ( $export_data as $item ) {
            $rows = array();
            foreach ( $item as $export_obj){
                $rows[] = iconv('utf-8', 'gbk//IGNORE', $export_obj);
//                 $rows[] = $export_obj;
            }
            fputcsv($fp, $rows);
        }

        // 將已經寫到csv中的數據存儲變量銷毀，釋放內存占用
           unset($export_data);
           ob_flush();
           flush();
    }
//     echo "<pre>";
//     print_r($rows);die;
    exit ();
?>