How to bold array elements in html table?

I'm trying to solve an exercise, but don't know how to go on. I need to make bold table's diagonal letters, like show on this picture:

enter image description here

Here's my code:

    <?php
    if (isset($_GET['word'])) {
        $word = strtolower($_GET['word']); 
        $arr = str_split($word);}
    else {
    }
    ?>

    <!DOCTYPE html>
    <html>
        <head>
            <meta charset="utf-8">
            <title>Tabel.</title>
            <style type="text/css">
                td {
                    position:relative;
                    width: 20px;
                    height: 20px;
                text-align: center;

            }
            }
                table {border-collapse:collapse;}
        </style>
    </head>
    <body>
        <form action="tabel.php" method="get">
        word: <input type="text" name="word"><input type="submit">
        </form>
        <?php
        if (isset($arr)){
            for ($i=0; $i < count($arr); $i++) {
                foreach ($arr as $taht) { ?>
                            <table border="1"                style="display:inline-table;">
                                <tr>
                                <td><?php echo $taht;?></td>
                                </tr>
                            </table>
                    </div><?php
                }
                ?> <br> <?php
            }
        }
        else {}

        ?>
    </body>
</html>

Solution

As i said in my very first comment :

for($i .....)
  for($j ....)
     if($i == $j)    echo "<b>.....