proxy.php

<?php

class Foo {
    private $woo;
    
    private $bar;
    
    public function __construct($woo)
    {
        $this->woo = $woo;
    }

    public function setBar($bar) { $this->bar = $bar; }
    
    public function getBar() { return $this->bar; }
    
    public function getWoo() { return $this->woo; }
    
    public function __clone() {
        $this->woo = clone $this->woo;
    }
}

$reflClass = new \ReflectionClass(Foo::class);

$fooProxy = $reflClass->newProxyInstance(
    new MyCustomInvocationHandler(new Foo($someWoo)), 
    [ExtraInferface::class]
);

/*
interface \ReflectionProxyInvocationHandler {
    public function invoke($objectInstance, $method, $arguments);
}

// However, Foo class properties, static members and methods are not copied over to anon class...
$fooProxy = new class(new MyInvocationHandler(new Foo($someWoo))) extends Foo implements ExtraInterface {
    private $invocationHandler;
    
    public function __construct(\ReflectionProxyInvocationHandler $invocationHandler)
    {
        $this->invocationHandler = $invocationHandler;
    }
    
    public static function __set_state(array $properties) { return $this->invocationHandler->invoke(null, __METHOD__, $properties); }
    public static function __callStatic($method, $args) { return $this->invocationHandler->invoke(null, $method, $args); }
    public function __call($method, $args) { return $this->invocationHandler->invoke($this, $method, $args); }
    public function __invoke() { return $this->invocationHandler->invoke($this, __METHOD__, func_get_args()); }
    public function __get($name) { return $this->invocationHandler->invoke($this, __METHOD__, func_get_args()); }
    public function __set($name, $value) { $this->invocationHandler->invoke($this, __METHOD__, func_get_args()); }
    public function __isset($name) { return $this->invocationHandler->invoke($this, __METHOD__, func_get_args()); }
    public function __unset($name) { $this->invocationHandler->invoke($this, __METHOD__, func_get_args()); }
    public function __clone() { $this->invocationHandler->invoke($this, __METHOD__, func_get_args()); }
    public function __sleep() { return $this->invocationHandler->invoke($this, __METHOD__, func_get_args()); }
    public function __wakeup() { $this->invocationHandler->invoke($this, __METHOD__, func_get_args()); }
    public function __toString() { return $this->invocationHandler->invoke($this, __METHOD__, func_get_args()); }
    public function __debugInfo() { return $this->invocationHandler->invoke($this, __METHOD__, func_get_args()); }
}
*/

assertTrue($fooProxy instanceof ExtraInterface);
assertTrue($fooProxy instanceof Foo);

$fooProxy->setBar(2); // Goes through InvocationHandler > Foo instance

assertTrue($fooProxy->getBar() === 2);