simple javascript/php or anything track youtube subscribe

I need you to build an html page with javascript that can track if someone has subscribe to my channel. case: i have an offer, i will send my ebook to web visitors's email address if they subscribed my youtube channel. so there will be a button , if a user clicked that button, a pop up will appear that ask the visitor to subscribe the channel example url : [login to view URL] after that the program will check wether the user has subscribed the channel. and if they have subscribed, then a download link will appear.