$max = 10;
$midPoint = ($max / 2) - 0.5;

for ($i = 0; $i < $max; $i ++)
{
  $rowLength = ceil(abs($i - $midPoint));
  $numStars = ($rowLength * 2) + ($max % 2 !== 0 ? 1 : -1);

  echo str_pad('*', $numStars, '*', STR_PAD_RIGHT) . "<br/>";
}