Coding a Website

Okay so I don't know where this goes but I guess it goes here because it has to do with my mod :/

so anyways im made a mod. MobDrops and I added code for a Donor Cape! so i was wondering how if anyone knows how to code HTML or PHP so that it would make a form asking for your minecraft username so when they pay for the cape they get redirected to a hidden link that has the form to put there user name in so they have the cape. I think its called like 'form to append a text file.' So like what they would do is put there mc username into a text box (Like OptiFine) and then hit submit. where it would put the line of code into the text file. the code is donors=(MCUserName) But they don't type donors= they just put their username and it automatically adds it to the front? If anyone know can you please tell me!? would really like to have the capes working! Thanks -JK