Request - Requests are sent to the API endpoint with the arguments using GET.
Response - The expanded Url is sent as response using php echo function.

Eg.
Request - https://www.linkexpander.com/?url=https://goo.gl/tzLLGm

Response - https://www.linkexpander.com

https://goo.gl/tzLLGm Redirects to https://www.linkexpander.com

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Note : You can use this as a free service to unshorten url's from your own API or website.
This can also be used to expand url's in bulk. PHP echo function is used to output final URL. You are not redirected to that link.