未定义的变量:con无法从mysql中获取数据

index.php

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

包括/nav.php

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

包括/db.php

<?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:''examplep''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:''examplep''htdocs''blog''includes''nav.php中为null

警告：mysqli_fetch_assoc（）要求参数1为mysqli_result，在第20行的C:''examplep''htdocs''blog''includes''nav.php中为null