Php 未定义变量：con无法从mysql获取数据

index.php

<?php include'includes/nav.php'; ?>
<?php include "includes/db.php"; ?>

<?php 
$allcategeories = mysqli_query($con,"SELECT * FROM categories");
while($row = mysqli_fetch_assoc($allcategeories ))
{
   echo "<li>{$row['cat_title']}</li>";
}
 ?>

<?php
$db['DBhost'] = "localhost";
$db['DBuser'] = "root";
$db['DBpass'] = "";
$db['DBNAME'] = "cms";
foreach($db as $key => $value) {
    define(strtoupper($key), $value);
}
$con = mysqli_connect(DBHOST,DBUSER,DBPASS,DBNAME);
if(!$con) {
    echo "cannot establish connection";
}
?>

包括/nav.php

<?php include'includes/nav.php'; ?>
<?php include "includes/db.php"; ?>

<?php 
$allcategeories = mysqli_query($con,"SELECT * FROM categories");
while($row = mysqli_fetch_assoc($allcategeories ))
{
   echo "<li>{$row['cat_title']}</li>";
}
 ?>

<?php
$db['DBhost'] = "localhost";
$db['DBuser'] = "root";
$db['DBpass'] = "";
$db['DBNAME'] = "cms";
foreach($db as $key => $value) {
    define(strtoupper($key), $value);
}
$con = mysqli_connect(DBHOST,DBUSER,DBPASS,DBNAME);
if(!$con) {
    echo "cannot establish connection";
}
?>

错误

注意：第19行C:\xampp\htdocs\blog\includes\nav.php中的未定义变量：con

警告：mysqli_query（）要求参数1为mysqli，在第19行的C:\xampp\htdocs\blog\includes\nav.php中为空

警告：mysqli_fetch_assoc（）要求参数1为mysqli_结果，在第20行的C:\xampp\htdocs\blog\includes\nav.php中为空

---

更改在index.php中包含文件的顺序

<?php include'includes/nav.php'; ?>
<?php include "includes/db.php"; ?>

<?php 
$allcategeories = mysqli_query($con,"SELECT * FROM categories");
while($row = mysqli_fetch_assoc($allcategeories ))
{
   echo "<li>{$row['cat_title']}</li>";
}
 ?>

<?php
$db['DBhost'] = "localhost";
$db['DBuser'] = "root";
$db['DBpass'] = "";
$db['DBNAME'] = "cms";
foreach($db as $key => $value) {
    define(strtoupper($key), $value);
}
$con = mysqli_connect(DBHOST,DBUSER,DBPASS,DBNAME);
if(!$con) {
    echo "cannot establish connection";
}
?>

首先包括数据库文件

<?php include "includes/db.php"; ?>

然后

因为你的数据库连接在你的

db.php

中被玷污了，包括在

nav.php

之后，这就是你得到的undeine变量con