Help setting textbox border colour on validation failure

<?php // RAY_form_highlight_errors.php
error_reporting(E_ALL);


// DEMONSTRATE HOW TO HIGHLIGHT ERRORS IN FORM INPUT
// CLIENT IS ASKED TO PUT IN A VALUE
// IF THE VALUE FAILS OUR TEST WE SHOW AN ERROR MESSAGE
// WE PUT A MARKER NEXT TO THE INPUT CONTROL ON THE FORM
// SEE http://www.w3schools.com/CSS/pr_class_visibility.asp


// THESE CONDITIONS ARE SET FOR THE INITIAL ENTRY
$error_any = 'hidden';
$error_abc = 'hidden';
$error_xyz = 'hidden';

// CAPTURE AND NORMALIZE THE POST VARIABLES, IF ANY
$abc = (isset($_POST["abc"])) ? trim(strtoupper($_POST["abc"])) : NULL;
$xyz = (isset($_POST["xyz"])) ? trim(strtoupper($_POST["xyz"])) : NULL;

// IF ANYTHING WAS POSTED, VALIDATE IT
if (!empty($_POST))
{
    // VALIDATE THE 'abc' FIELD
    if ($abc != 'ABC')
    {
        $error_any = 'visible';
        $error_abc = 'visible';
        $abc       = NULL;
    }

    // VALIDATE THE 'xyz' FIELD
    if ($xyz != 'XYZ')
    {
        $error_any = 'visible';
        $error_xyz = 'visible';
        $xyz       = NULL;
    }

    // DO WE HAVE INPUT FREE FROM ANY ERRORS?
    if ($error_any != 'visible')
    {
        echo "CONGRATULATIONS";
        die();
    }
}

// IF NOTHING WAS POSTED, OR IF THERE ARE ERRORS, WE NEED NEW CLIENT INPUT
$form = <<<ENDFORM
<style type="text/css" media="all">
.error_any { visibility:$error_any; }
.error_abc { visibility:$error_abc; }
.error_xyz { visibility:$error_xyz; }
</style>
<pre>
<form method="post">
<span class="error_any">PLEASE CORRECT THE FOLLOWING ERRORS</span>
<span class="error_abc">YOU MUST ENTER 'abc' IN THIS FIELD</span>
PLEASE ENTER "ABC" HERE: <input name="abc" value="$abc" />
<span class="error_xyz">YOU MUST ENTER 'xyz' IN THIS FIELD</span>
PLEASE ENTER "XYZ" HERE: <input name="xyz" value="$xyz" />
<input type="submit" />
</form>
ENDFORM;

// WRITE THE FORM WITH THE APPROPRIATE CSS STYLES ON THE ERROR MESSAGE FIELDS
echo $form;

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