PHP Code:
<html>
<head>
<title>Demo</title>
</head>
<body>
<form method="post" action="">
<label for="header_text_option">
Display Header?
</label>
<input type="checkbox" id="header_text_option" name="header_text_option" />
<?php if(array_key_exists('header_text_option', $_POST)): ?>
<label for="header_text">
Enter text to display in header
</label>
<input type="text" name="header_text" value="Enter Header Text" />
<?php endif; ?>
<input type="submit" value="Submit" />
</form>
</body>
</html>

Edit: Bah, too slow!
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