在symfony中将内容类型设置为json

I am using symfony 1.4, to create my project with propel as ORM. i want to get the response in JSON format, when i call a url. I have set the headers to "application/json" but it is not working, i am getting the response back in HTML format..which i am not able to decode. How can we set content-type in symfony?? example code: Action- I am using symfony 1.4, to create my project wi