Extension.php

<?php

class Extension implements IFacExtension
{
    public function add()
    {
        
    }
    public function read()
    {
        
    }
    public function update($id)
    {    
        
    }
    
    public function remove($id)
    {
        
    }
}

FacExtension.php

<?php

class FacExtension
{
    public $extension = null;
    
    public function __construct(IFacExtension $extension)
    {
        $this->extension = $extension;
    }
    
    public function add()
    {
        return $this->extension->add();
    }
    
    public function read($id)
    {
        return $this->extension->read($id);
    }
    
    public function update($id)
    {
        return $this->extension->update($id);
    }
    
    public function remove($id)
    {
        return $this->extension->remove($id);
    }
}

IFacExtension.php

<?php

interface IFacExtension
{   
    public function add();
    public function remove($id);
    public function update($id);
    public function read($id);
}

index.php

<?php

include('IFacExtension.php');
include_once('FacExtension.php');
include_once('Extension.php');


$facext = new FacExtension(new Extension());

if ($facext->add()) {
    echo 'Saved';
}