在执行PHP时，我得到空值

当我在PHP中的“$result”中选择3个值时，我得到最后三行值“NULL”

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do array\u filter（$relen）和array\u filter（$relen1）您正在循环的这些数组中可能有空元素。\n不应将用户输入连接到SQL查询中，应使用以避免SQL注入do array\u filter（$relen）和array\u filter（$relen1）您正在循环的这些数组中可能有空元素。\n不应将用户输入连接到SQL查询中，应使用以避免SQL注入
<?php
include "seminit.php";
$result1 = $_POST["result"];
$semno = $_POST["semno"];
$slotno= $_POST["slotno"];
$tdat=date("d/m/Y")."  /  ".$slotno;
$sql1="alter table $semno add `".$tdat."` varchar(30)";
$conn->query($sql1);
$arr = (explode(",",$result1));
$len = count($arr);
$query = "SELECT name FROM $semno";
$res = $conn->query($query);
$array = array();
if (mysqli_num_rows($res)>0)
{
    //$response["data"] = array();    
    while ($row = mysqli_fetch_array($res)) 
   {
        $array[] = $row["name"];
        //array_push($response["data"], $user);
    }
}
$len1 = count($array);
$res=array_intersect($arr,$array);
$res1=array_diff($array,$arr);
print_r($res1);
print_r($res);
$relen=count($res);
$relen1=count($res1);
$present="Present";
$absent="Absent";
for($i=0;$i<$relen;$i++)
{
    $sql = "update $semno set `$tdat`='$present' where `name`='$res[$i]'";
    $conn->query($sql);
}
for($j=0;$j<$relen1;$j++)
{
    $sql = "update $semno set `$tdat`='$absent' where `name`='$res1[$j]'";
    $conn->query($sql);
}
$response["success"] = array();
$response["success"] = 1;
echo(json_encode($response));
?>