how to use firebase or other solutions to sign document on phone from web?

I want to develop a service: a client who wants to do a transaction, have to sign it electronically with his phone; So when he wants to do complete the transaction a pop-up gives him the bouton to sign the form. when he push sign, a notification appears on his phone then he enters his pin and does the signature on his phone then-then the transaction is competed.

I don't know how exactly I can do this, so for the form, i will use PHP, and for the mobile app I will use Java, but I don't know how to do a relation between them. Someone tells me to use firebase for this but I don't know how!

Please give me some guidance and solutions to do this for my semester project in university!

Thank you.