Simple PHP code

This is a simple project and I know almost every php programmer knows how to do it in 5 minutes. I want a simple text field where a person enters a text and it will replace a string in another text field. For example : 1. A user enters "myusername" in the text field. 2. I will have another large field with other texts and a string called "<username>". 3. When a user clicks submit then it will replace <username> with "myusername". Its that simple . I am looking for 30 and lower please. I know you can get the code from another site or create it in 5 minutes.