金字塔结构怎么能分裂成两组小金字塔（内有图例）？

$a = array(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30);echo '<div align=center>';for($i=0,$n=2; $i<count($a); $i+=$n,$n*=2) {  echo join(' ', array_slice($a, $i, $n)), '<br>';}echo '</div>';for($i=0,$n=2; $i<count($a); $i+=$n,$n*=2) {  list($r[], $l[]) = array_chunk(array_slice($a, $i, $n), $n / 2);}echo '<div align=center>';foreach($r as $v) echo join(' ', $v), '<br>';echo '</div>';echo '<div align=center>';foreach($l as $v) echo join(' ', $v), '<br>';echo '</div>';