You could use the Document class to add those facebook tags (as many as you want). Just add two extra methods setFacebookDescription and getFacebookDescription, so you have to add the followings:
<?php
class Document {
private $facebook_description;
public function getFacebookDescription() {
return $this->facebook_description;
}
public function setFacebookDescription($facebook_description) {
$this->facebook_description = $facebook_description;
}
}
On each controller, you will find at the end of each method, a call which loads the header of Opencart, something like this $data['header'] = $this->load->controller('common/header'); (example). Note that it might differ from yours, it depends on Opencart version.
Now, in header.php controller you add:
<?php
class ControllerCommonHeader extends Controller {
public function index() {
$data['facebook_description'] = $this->document->getFacebookDescription();
}
}
this will get the facebook_description variable and pass it to the view header.tpl. Next, add the facebook tags between your <head> tags in header.tpl file:
<!DOCTYPE html>
<head>
<?php if ($facebook_description != '') { ?><meta property="og:description" content="<?php echo $facebook_description; ?>" /><?php } ?>
</head>
Finally, you can set facebook_description in each controller, by calling $this->document->setFacebookDescription('my description');.
Example: in product.php controller you add
<?php
class ControllerProductProduct extends Controller {
public function index() {
// code...
$this->document->setTitle($product_info['meta_title']);
$this->document->setDescription($product_info['meta_description']);
$this->document->setFacebookDescription($product_info['meta_description']);
// the rest of the code...
}
}
here you set the $product_info['meta_description'] as facebook description tag, however you could also use $product_info['name'] or other variable.
Final note: you can change all the Opencart system classes with the vqmod.
