I would use the Rules module to get this to work, using a variation of the sample rule included in my answer to "How to use the Rules module to implement a custom redirect for an outdated URL?". Here are some modifications you'd need to get it to work for your case also:
- Adapt the URLs to point to your checkout page.
- Add a Rules event specific to the social site they used to perform a login, i.e.:
By using Rules as detailed above, you probably won't need the custom code anymore that you already have (as mentioned in your question).
To make the value of the URL argument available as a new variable to Rules, you can use the Rules URL Argument module. Here is a quote about it (from its project page):
... provides two rules conditions based on URL arguments:
- check if a URL argument is present.
- compare the value of an URL argument.
It also provides an action that makes the value of an URL argument available as new variable to Rules.
The newly provided Rules actions and conditions can be found under the "URL Argument" conditions and actions groups in the rule configuration interface.
For an example of how to use it, refer to Comment # 3 in issue # 1686360 which is about "Pass an amount to the url, ie; site/content/node-title?amount=10 or /node-title?=amount:10". Here is the relevant part of it:
- add a "Check if URL argument exist" condition to your rule and set the "ARGUMENT" value to "amount".
- add "Provide URL argument value" action to the rule and set "URL ARGUMENT NAME" value to "amount" and in the "Provided variables" section "ARGUMENT FROM URL - Variable name" to e.g. "
You will now be able to use the amount value as a variable "
amount-from-url" for other actions in this rule.
PS: If you're not familiar (enough/yet) with Rules, checkout the 32 (!!!) great, and free, video tutorials Learn the Rules framework, organized in 7 chapters.