重点是：红色部分的格式

$sql = "INSERT INTO data_type (title, admin_name,flid) VALUES ('".$title."','".$admin_name."',1)";

<?php 
if($title){
$admin_name=$_SESSION['username'];  
//$time=date();
    $sql = "INSERT INTO data_type (title, admin_name,flid) VALUES ('".$title."','".$admin_name."',1)";
    
    if (mysqli_query($conn, $sql)) {
      echo "新记录插入成功";
    } else {
      echo "Error: " . $sql . "<br>" . mysqli_error($conn);
    }
    
    mysqli_close($conn);
}

?>