PHP OOP Database Connection using Seperate Config File with Clasess and functions

The class Dbh is not getting the variables from class ConfigApp.

Everypage.php in the head:

<?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(); 
        }
    }
}
?>

Variables from config.inc.php are not being passed into dbh.classes.php.

In the ConfigApp class you've defined Class Constants, not properties. Therefore the syntax to access them is different, as that documentation explains. The pattern is:

self::constantname

So in your case, you'd need

self::dbhost

etc.

Live demo: https://3v4l.org/XjCeU

Therefore your code in connect() would be

$dsn = 'mysql:host=' .self::dbhost . ';dbname=' . self::dbname;
$pdo = new PDO($dsn, self::dbuser, self::dbpass);

P.S. Everything after return $pdo; in the connect() function is redundant, since code after a return cannot be executed, and in any case you don't need to define the connection twice!

Also, there seems to be no need to pass in new database connection values through the constructor of the Dbh class, since you're intending to read them from the ConfigApp class constants.