HTML Tags Counter

<!--Write a PHP script HTMLTagsCounter.php which generates an HTML form like in the example below. It should contain a label, an input text field and a submit button. The user enters HTML tag in the input field. If the tag is valid, the script should print “Valid HTML tag!”, and if it is invalid – “Invalid HTML Tag!”. On the second line, there should be a score counter. For every valid tag entered, the score should increase by 1.
Hint: You may use sessions. Use an array to store all valid HTML5 tags.-->

<!DOCTYPE html>
<html>
<head lang="en">
    <meta content="text/html" charset="UTF-8">
    <title>HTML Tags Counter</title>
</head>
<body>
<form action="#" method="post">
    <p><label for="htmlTag">Enter HTML tags:</label></p>

    <p><input type="text" name="htmlTag" id="htmlTag"/>
        <input type="submit" value="Submit"/>
    </p>
</form>
<?php
session_start();
if ($_POST && array_key_exists('htmlTag', $_POST)) {
    $validHTML5Tags = array('!--...--', '!DOCTYPE', 'a', 'abbr', 'address', 'area', 'article', 'aside', 'audio',
        'b', 'base', 'bdi', 'bdo', 'blockquote', 'body', 'br', 'button', 'canvas', 'caption', 'cite',
        'code', 'col', 'colgroup', 'data', 'datalist', 'dd', 'del', 'details', 'dfn', 'dialog',
        'div', 'dl', 'dt', 'em', 'embed', 'fieldset', 'figcaption', 'figure', 'footer', 'form',
        'h1', 'h2', 'h3', 'h4', 'h5', 'h6', 'head', 'header', 'hgroup', 'hr', 'html', 'i', 'iframe',
        'img', 'input', 'ins', 'kbd', 'keygen', 'label', 'legend', 'li', 'link', 'main', 'map',
        'mark', 'menu', 'menuitem', 'meta', 'meter', 'nav', 'noscript', 'object', 'ol', 'optgroup',
        'option', 'output', 'p', 'param', 'pre', 'progress', 'q', 'rb', 'rp', 'rt', 'rtc', 'ruby',
        's', 'samp', 'script', 'section', 'select', 'small', 'source', 'span', 'strong', 'style',
        'sub', 'summary', 'sup', 'table', 'tbody', 'td', 'template', 'textarea', 'tfoot', 'th',
        'thead', 'time', 'title', 'tr', 'track', 'u', 'ul', 'var', 'video', 'wbr');


    if (!array_key_exists('counter', $_SESSION)) {
        $_SESSION['counter'] = 0;
    }

    $checkTag = 'Invalid';
    if (in_array($_POST['htmlTag'], $validHTML5Tags)) {
        $checkTag = 'Valid';
        $_SESSION['counter'] += 1;
    }

    echo "<p>$checkTag HTML tag!</p>\n<p>Score: " . $_SESSION['counter'] . "</p>\n";
}
?>
</body>
</html>