binary-trees PHP #2 program

<?php 
/* The Computer Language Benchmarks Game
   https://salsa.debian.org/benchmarksgame-team/benchmarksgame/

   contributed by Peter Baltruschat
   modified by Levi Cameron
   modified by Craig Russell
   *reset*
*/

class Tree {
   public $l;
   public $r;
   
   public function __construct($depth) {
      if($depth) {
         $this->l = new Tree(--$depth);
         $this->r = new Tree($depth);
      }
   }
   
   public function check() {
      return 1
         + ($this->l->l === null ? 1 : $this->l->check())
         + ($this->r->l === null ? 1 : $this->r->check());
   }
}

$minDepth = 4;

$n = $argc == 2 ? $argv[1] : 1;
$maxDepth = $minDepth + 2 > $n ? $minDepth + 2 : $n;
$stretchDepth = $maxDepth + 1;

$stretch = new Tree($stretchDepth);
printf("stretch tree of depth %d\t check: %d\n",
   $stretchDepth, $stretch->check());
unset($stretch);

$longLivedTree = new Tree($maxDepth);

$iterations = 1 << $maxDepth;
do
{
   $check = 0; 
   for($i = 1; $i <= $iterations; ++$i)
   {
      $check += (new Tree($minDepth))->check();
   }
   
   printf("%d\t trees of depth %d\t check: %d\n",
      $iterations, $minDepth, $check);
   
   $minDepth += 2;
   $iterations >>= 2;
}
while($minDepth <= $maxDepth);

printf("long lived tree of depth %d\t check: %d\n",
   $maxDepth, $longLivedTree->check());
    

NOTES:
64-bit Ubuntu quad core
PHP 7.4.5 (cli) (built: May 11 2020 13:25:48) ( NTS )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies


Mon, 11 May 2020 20:45:49 GMT

COMMAND LINE:
/opt/src/php-7.4.5/bin/php -n -d memory_limit=4096M binarytrees.php-2.php 21

PROGRAM OUTPUT:
stretch tree of depth 22	 check: 8388607
2097152	 trees of depth 4	 check: 65011712
524288	 trees of depth 6	 check: 66584576
131072	 trees of depth 8	 check: 66977792
32768	 trees of depth 10	 check: 67076096
8192	 trees of depth 12	 check: 67100672
2048	 trees of depth 14	 check: 67106816
512	 trees of depth 16	 check: 67108352
128	 trees of depth 18	 check: 67108736
32	 trees of depth 20	 check: 67108832
long lived tree of depth 21	 check: 4194303