PHP function echoes unexpected HTML value

I have a piece of php code inside html tag which is supposed to change the tag's style in accordance with the contents of the URL.

There is an html login form which looks like this:

<form class="userdata" action="login.php" method="post">
    <input type="text" name="email" placeholder="E-mail" <?php fillin('email'); enlight_unfilled('email');?>><br>
    <input type="password" name="pwd" placeholder="Password"><br>
    <button type="submit" name="login-submit">Login</button>
</form>

Here are the functions fillin and enlight_unfilled:

<?php
function fillin($key) {
    if (isset($_GET[$key])) echo "value=".$_GET[$key];
    else echo NULL;
}

function enlight_unfilled($key) {
    if (isset($_GET['error']))
        if (isset($_GET[$key]) and $_GET[$key] !== "") echo NULL;
        else echo "style='border-color: red'";
    else echo NULL;
}
?>

If I only apply one of the functions within the tag, they both do what they are expected to – either save the email in the field if it has been already typed in or enlighten the email field if it has been left empty. But if I apply them together, when the field is empty, php assigns the field value 'style='border-color:. I also tried to use functions like print and printf, but the result is the same:

I am a beginner at php coding and mixing it with html, so the question may appear to be dumb, but I did not manage to find any sort of a solution to this issue, so thanks for help and patience in advance!