PHP未连接到数据库

所以我有一个带有表单的html页面，该页面链接到数据库，但是每当我尝试向数据库中插入内容时，我得到的都是404错误，说该链接不存在。这是PHP代码：

<?php
 include_once 'dbh.inc.php';

 $nume = $_POST['user_name'];
 $CNP= $_POST['user_cnp'];
 $domiciliu = $_POST['user_domiciliu'];
 $Str = $_POST['user_str'];
 $Bl = $_POST['user_bl'];
 $Sc = $_POST['user_sc'];
 $Ap = $_POST['user_ap'];
 $Jud = $_POST['user_jud'];
 $CI = $_POST['user_CI'];
 $Seria = $_POST['user_serie'];
 $Nr = $_POST['user_nr'];
 $Avocat = $_POST['user_avocat'];
 $Inspectorat = $_POST['user_inspectorat'];
 $Plangere_Contraventionala = $_POST['user_plangere'];
 $Nr_Contraventie = $_POST['user_nr_s'];
 $Serie_Contraventie = $_POST['user_serie_s'];
 $Data_Contraventie = $_POST['trip-start'];
 $Data_Comunicare = $_POST['trip-start_2'];
 $Suma_RON = $_POST['user_RON'];
 $Motive_Rez = $_POST['user_motiv'];
 $Serie_Sanctiune = $_POST['user_serieS'];
 $Sanctiune = $_POST['user_list'];
 $Acord_Semnatura = $_POST['user_acord'];
 $Cuantum_Amenda = $_POST['user_A_RON'];
 $Art = $_POST['user_art'];
 $Martori = $_POST['user_martori'];

$sql="INSERT INTO  users ( nume, cnp, domiciliu, Str, Bl, Sc, Ap, Jud , CI, Seria, Nr, Avocat, Inspectorat, Plangere_Contraventionala, Nr_Contraventie, Serie_Contraventie, Data_Contraventie,
 Data_Comunicare, Suma_RON, Motive_Rez, Serie_Sanctiune, Sanctiune, Acord_Semnatura, Cuantum_Amenda, Art, Martori)
VALUES ('$nume','$CNP', '$domiciliu', '$Str', $Bl, $Sc, $Ap, '$Jud' , '$CI', '$Seria', '$Nr', $Avocat, '$Inspectorat', '$Plangere_Contraventionala', '$Nr_Contraventie', '$Serie_Contraventie', '$Data_Contraventie',
 '$Data_Comunicare', $Suma_RON, '$Motive_Rez', '$Serie_Sanctiune', '$Sanctiune', '$Acord_Semnatura', $Cuantum_Amenda, $Art, '$Martori');";
mysqli_query($conn,$sql);

header("Location: main.php?signup=success");


?>

这就是打开该文件的方式：

<form action="signup.inc.php" method="post">
        <ul>
         

这两个文件都在同一个文件夹中，我检查了名称，它们确实匹配。

如果这也很重要，这里是“ dbh.inc.php”：

<?php

$dbServerName = "localhost"; 
$dbUserName = "root";
$dbPassword = "" ;
$dbName = "user";

$conn = mysqli_connect($dbServerName, $dbUserName, $dbPassword, $dbName);