I want PHP script which counts how many times an item has been bought, increasing the counter by one every time the PHP script (of the purchase) has been executed, and showing it in the page database.
Let's say, when the user is redirected back to the site via Paypal they load the page thankyou.php which on load, increases the database value (of the counter) in a table by one.
I couldn't find any examples so I'm hoping you kind guys here could help me out.
php is server side language that is executed on sever side before the page loads. in your case you have to make ajax call to trigger the php code.
Update: mysql_query("UPDATE table_name SET id = id + 1 WHERE id = '$id'");
You do not need to increment the value in PHP, you can do it in your query directly in your query:
UPDATE hits_log SET hits = hits+1 WHERE page = *somthing*
This can be executed using php's MySQLi library, or similar.