selection in form-field

i have quite an easy question: is there a way in PHP to trace the selection in a certain form-field? i need this for building a simple html-edit application, which allows a user to select his/her text within a field, and then click 'bold' for instance..the PHP should insert <b> before the selection, and </b> after the selection. 1) should i use PHP or is this a typical JavaScript-thing? (or a combination?) 2) if you could help with a function or small script I would me most grateful grace michiel