In your HTML, add the input field like this:
<input type="text" name="username" value="<?php echo htmlspecialchars($_GET['username']); ?>" />
Basically, the value attribute of the text field needs to be set to:
<?php echo $_GET['username']; ?>
The code right above this is how you would output a get variable in php whether you are putting it in a text field or not. To access get variables, always use:
Then you can assign it to variables or pass it as a function parameter.
**However, I strongly do not recommend passing sensitive information like usernames and passwords through GET variables. **
First off, users could change the URL hence changing the variable. They could also accidentally share the URL with someone and that could give someone else access to their account. I would recommend that you create a cookie on their machine that is set to a random ID, and then in a MySQL database, associate that ID with a username so that you know the user can't accidentally share their account or change their username through the URL.