Is voice Over Ip posible in PHP

hi, We have a web based text chat application using php, apache and mysql. ( It works on the principle that each client inserts the chat content in the db and the common updated contents are retrieved by contanlty requesting a php page using browser's auto refresh and displaying the chat contents) Now my question is that, is possible to add voice chat in this application using php (or other languages), so that clients need not install any software. Please give some ideas or suggestions to implement the voice chat in our application. Regards