repeat get set in function

main.php

<?php
// create a class User with :
//                      name
//                      age
// create a class Customers extended from User
// create a method pay() that simulate a customers paying

Class User {
    protected $name;
    protected $age;

    public function __construct($name, $age){
       $this->age = $age;
       $this->name = $name;
    }

    public function __get($property){

       if(!property_exists($this, $property)){
           echo $property . " does not exist in " . $this;
           return NULL;
       }
       return $this->$property;
    }

    public function __set($property, $value){

        if(gettype($value) != gettype($property)){
            echo "can not affect something of type " . gettype($value) . " into " . $property . " of type "
                 . gettype($property);
            return NULL;
        }

        if(!property_exists($this, $property)){
            echo $property . " does not exist in " . $this;
            return NULL;
        }

        $this->$property = $value;
    }
}

Class Customers extends User{
    private $balance;
    public function __construct($name, $age, $balance)
    {
        parent::__construct($name, $age);
        $this->balance = $balance;
    }

    public function __get($property){
        if(!property_exists($this, $property)){
            echo $property . " does not exist in " . $this;
            return NULL;
        }
        return $this->$property;
    }
    public function __set($property, $value){

        if(gettype($value) != gettype($property)){
            echo "can not affect something of type " . gettype($value) . " into " . $property . " of type "
                 . gettype($property);
            return NULL;
        }

        if(!property_exists($this, $property)){
            echo $property . " does not exist in " . $this;
            return NULL;
        }

        $this->$property = $value;
    }


    public function pay($value){
        if($this->balance - abs($value) < 0){
            echo "Insufficient Balance";
            return NULL;
        }

        if(!is_numeric($value)){
            echo $value . "is not a  number";
            return NULL;
        }
        $this->balance = $this->balance - $value;
        echo $this->name . " paid $ " . $value . "<br>";
        echo $this->name . " have a balance of $ " . $this->balance . "<br>";

    }

}

$customer = new Customers("Yolan", 22, 10);
$customer->pay(4);