Delete a character from a string variable on key pressing event?

First of all I am newbie in web development, I will be as specific as I can:

The idea is I generate a random string variable for example "asdfg" and I print out inside a html or php page. When the user presses 'a' chararcter on the keyboard, the script deletes the 'a' character from the "asdfg" so the variable changes to "sdfg", then print out this new string, of course replace the old string. And so on, the user presses 's' chararcter, deletes the 's' from "sdfg", it will be "dfg", print out. Delete only when the right character was pressed.

If I can implement the code to be in a loop, so I can generate more words (strings) to play with, that would be awesome.

The script language could be anything, but please suggest a newbie friendly solution.