php用csv导出大数据

header ( "Content-type:application/vnd.ms-excel" );
header ( "Content-Disposition:filename=" . iconv ( "UTF-8", "GB2312//IGNORE", "查询用户列表" ) . ".csv" );
$out = $column_name;
echo iconv ( "UTF-8", "GB2312//IGNORE", $out );

$pre_count = 20000;
for ($i=0;$i<intval($total_export_count/$pre_count)+1;$i++){
$export_data = $db->getAll($sql." limit ".strval($i*$pre_count).",{$pre_count}");
foreach ( $export_data as $key => $item ) {
echo iconv ( "UTF-8", "GB2312//IGNORE", "\n".implode(',',$item) );
}

// 将已经写到csv中的数据存储变量销毁，释放内存占用
unset($export_data);
}

exit ();