Bagaimana Membuat Segitiga bintang dengan PHP?

<table>
<?php
function td($isi, $warna)
{
    echo "<td align='center' style='width:20px;height:20px;border:1px solid black;background-color:".$warna.";color:white'>".$isi."</td>";
}
$n = 9;

for($a=1;$a<=9;$a++)
{
    echo "<tr>";
    for($b=1;$b<=$n;$b++)
    {
        if(($b<=$a && $b+$a<=$n+1) || ($b>=$a && $b+$a>$n))
        {
            td("*", "black");
        }
        else
        {
            td("", "white");
        }
    }
    echo "</tr>";
}
?>
</table>

test https://tehplayground.com/SQ2lW5EkE92bTOdh klik save and run