This question already has an answer here:
I have a github page in which there's an "a" tag (). When the user clicks on it i want them to be redirected to a php page, which is in another repository. Which path should i put in "href"?
On local this works
<a href="http://localhost/index1.php"
onclick="window.open(this.href); return false">Use the online tool.</a>
</div>
GitHub gh-pages will not execute PHP code. It only handles static content for now.