Error Messages

Going by the names, this seems to be an online shop or similar software with actual customers. In that case, error messages should be factual and non-insulting. You cannot know if this was an attacker, or someone who might just have made a mistake.

Handling Errors

Contrary to your comment, catching an exception, printing the message, and then continuing the script as if nothing happened is not handling an error. If you cannot recover from it, redirect to a custom error page or similar.

Style

Use correct indentation. This is really important and improves readability of your code a lot.

Request Types

Be consistent with your request types. Right now, you are checking GET, but retrieving REQUEST. So an attacker could post the pid value and get around your checks (which isn't that bad because you are using prepared statements, but still).

Return Early

You can reduce if nesting by returning early:

if ( $id == 0 ) {
    echo "Only numeric values allowed";
    return;
}

Loading Config Data

You are loading all config entries into an array, but you are only using frontend_title. It would be more efficient to only load what you actually need.

XSS

You are not sanitizing your data with htmlspecialchars, so anyone creating frontend titles, product titles, or product descriptions will be able to execute arbitrary Javascript on the clients machines, and can thus steal cookies, log passwords, deface the website, etc.

Several things...

Indentation

This:

if ( $id != 0 ) {
try {
// etc.
} catch (PDOException $exception) { // to handle error
    echo "Error: " . $exception->getMessage();
}
    }

Would read easier as such:

if ( $id != 0 ) {
    try {
        // etc.
    } catch (PDOException $exception) { // to handle error
        echo "Error: " . $exception->getMessage();
    }
}

Comments

Many of your comments are of very little use, when the code speaks for itself. For instance:

    } catch (PDOException $exception) { // to handle error

Or:

// select all data

Or even:

// store retrieved row to a variable

SQL

You will save yourself and Mr. or Ms. Maintainer a good deal of grief if you format your SQL in a way that SQL programmers will be familiar with. For instance this:

$query = "SELECT p.product_id AS product_id, p.product_title AS product_title, p.product_description AS product_description, categories.category_name AS category_name, p.product_category AS product_category, p.product_image AS product_image, p.product_image_thumb AS product_image_thumb FROM products p LEFT JOIN categories ON p.product_category=categories.category_id WHERE p.product_id = ? LIMIT 1";

Could instead be:

$query = "SELECT 
            p.product_id AS product_id, 
            p.product_title AS product_title, 
            p.product_description AS product_description, 
            categories.category_name AS category_name, 
            p.product_category AS product_category, 
            p.product_image AS product_image, 
            p.product_image_thumb AS product_image_thumb 
        FROM products p 
        LEFT JOIN categories ON p.product_category=categories.category_id 
        WHERE p.product_id = ? LIMIT 1";

I will say that it is good that you are using PDO. More experienced PHP programmers will hopefully have more to say about PHP code, I'm mostly a SQL guy.