隔多少插一条数据

public function insertcheckemail(){
$table = 'ztest';
$n = 0;


$total=M($table)->count();
echo $total;
for($i=0; $i<=$total; $i=$i+5){
echo ('<br>'.$i."<br/>$i,1");
$c=M($table)->limit("$i,1")->select();
$c=$c[0];
//var_dump($c);
$id=$c['id'];
unset($c['id']);
if($c['test']==0){

M($table)->data($c)->add();
$c['test'] = 1;
if($n%2){
$c['email'] = '';
}else{
$c['email'] = '';
}
M($table)->where(array('id'=>$id))->data($c)->save();
echo M($table)->getLastSql();
//M('contact')->data($c)->add();
}