How to change to content of the textarea with php !

I'm using spip in php3, it's some kind of forum , and when you edit an article , like what i'm doing , there is a textarea , to input
I want to do some thing more with it , like insert smiley , or something more animate , and the most important i want to do
is to use vietnamese language inside this textarea , it means , when you type "as" , it will change into a different char .
That means i have to be able to change the content of the textarea when you type any char into the text area . Or use another
kind of textarea , that have the methode to do that ?
I know this problem is too far old , people do that so easy . But i don't know where to begin , i think there's must be something
that can help me , i don't want to start at zero , and think out all the thing that people already have done . Please show me
the link to the problem i want , i don't know where to find it at all . Thanks so much .

It depends how you want to do things, when you want to replace stuff while you type, you have to use JS. When you want to replace some characters into different ones after submitting your form, check out PHP.net. They got loads of information about it. Search for the str_replace() function and continue from there.

Good luck & Cheers,