2D Array Spiral.php

<?php

$input = 9;

// 1x1 => 1
// 2x2 => 4
// 3x3 => 9
// 4x4 => 16

/*
i:0 1 2  
0  1 8 7
1  2 9 6
2  3 4 5

2x2
1 4 
2 3

9x9
01 32 31 30 29 28 27 26 25 
02 33 56 55 54 53 52 51 24 
03 34 57 72 71 70 69 50 23 
04 35 58 73 80 79 68 49 22 
05 36 59 74 81 78 67 48 21 
06 37 60 75 76 77 66 47 20 
07 38 61 62 63 64 65 46 19 
08 39 40 41 42 43 44 45 18 
09 10 11 12 13 14 15 16 17 

**/

$size = $input * $input;
$result = [];
$counter = 1;

/*
 1 -> towrds right, bottom
 0 -> stays 
-1 -> towards left or top
**/ 
$di = 1;
$dj = 0;

$i = 0;
$j = 0;

while ($counter <= $size) {
  $result[$i][$j] = $counter++;
  
  $i += $di;
  $j += $dj;
  
  // checks for boundries
  if (invalid($i, $j, $input, $result)) {
    // reset first
    $i -= $di;
    $j -= $dj;
    
    // change direction
    if ($di == 1 && $dj == 0) {
      $di = 0;
      $dj = 1;
    } elseif ($di == 0 && $dj == 1) {
      $di = -1;
      $dj = 0;
    } elseif ($di == -1 && $dj == 0) {
      $di = 0;
      $dj = -1;
    } elseif ($di == 0 && $dj == -1) {
      $di = 1;
      $dj = 0;
    }
    
    // adds up again
    $i += $di;
    $j += $dj;
  }
  
}

$maxWS = ceil(log10($size+1));

for ($i = 0; $i < $input; $i++) {
  for ($j = 0; $j < $input; $j++) {
    echo str_pad($result[$i][$j], $maxWS, "0", STR_PAD_LEFT) . " ";
  }
  echo PHP_EOL;
}


function invalid($i, $j, $size, $result) 
{  
  return $j < 0 || $i < 0 || $j >= $size || $i >= $size || isset($result[$i][$j]) != false;
}
 
?>