Add class in HTML with PHP

I have a form with email and password validation. When password is wrong the box password has to be red to show that something go wrong, i do that adding a "vermei" class.

I have to do this in PHP, already attempted:

$dom=new DOMDocument;
$dom->loadHTML($html);
$input=$dom->getElementById('password');
foreach($input as $pd){
    $pd->setAttribute('class', $pd->getAttribute('class').' vermei');
}
$html = $dom->saveHTML(); // return 'variable $html not found'

and

echo '<script>
        alert("Password Incorrect!");
        history.go(-1); //work until this part, the code below do not run. 
 //I did this go back cuz when form is submitted, go to config.php, and
 // this code back to sign in page
        var password = document.getElementById("password");
        document.getElementById("password").focus();
        password.classList.add("vermei");
    </script>';

Someone can help me with adding a "vermei" class in Password input?

Solution 1:

Your first code doesn't work because $input is a single element, not a list, so you can't loop over it. Replace the loop with just:

$input->setAttribute("class", $input->getAttribute("class") . " vermei");

Normally you would do this in the code that outputs the form, e.g.

<form>
    ...
    Password: <input id="password" type="password" name="password" <?php if ($password_incorrect) echo 'class="vermei"'; ?>>
    ...
</form>