subracks.php

<?php

$myrack = "AAABNN";
$racks = [];
for($i = 0; $i < pow(2, strlen($myrack)); $i++){
	$ans = "";
	for($j = 0; $j < strlen($myrack); $j++){
		//if the jth digit of i is 1 then include letter
		if (($i >> $j) % 2) {
		  $ans .= $myrack[$j];
		}
	}
	if (strlen($ans) > 1){
  	    $racks[] = $ans;	
	}
}

$racks = array_unique($racks);

print_r($racks);

?>