Dbh类未从ConfigApp类获取变量。
Everypage.php在头部：

<?php include 'config.inc.php'; ?>

config.inc.php:

<?php
class ConfigApp
{
    const dbhost = "localhost";
    const dbname = "secretdbname";
    const dbuser = "secretdbsuer";
    const dbpass = "secretdbpass";
    const var1 = "something";
    const var2 = "nothingToDoWithDB";
    const var3 = "something else else";
}
?>

dbh.class.php:

<?php
class Dbh extends ConfigApp{
    public function __construct($dbhost, $dbname, $dbuser, $dbpass) {
        $this->dbhost = $dbhost;
        $this->dbname = $dbname;
        $this->dbuser = $dbuser;
        $this->dbpass = $dbpass;
    }

    protected function connect() {
        try {
            $dsn = 'mysql:host=' .$this->$dbhost . ';dbname=' . $this->dbname;
            $pdo = new PDO($dsn, $this->dbuser,$this->dbpass);
            $pdo->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC);
            return $pdo;
            
            $dbh = new PDO('mysql:host=localhost; dbname=c1gnsoftdboop' , $dbuser, $dbpass);
            return $dbh;
        } catch (PDOException $e) {
            print "Database Connection Error!: " . $e->getMessage() . "<br/>"; 
            die(); 
        }
    }
}
?>

config.inc.php中的变量没有被传递到dbh.classes.php中。