Spiral matrix traversal in PHP (see in action http://sandbox.onlinephpfunctions.com/code/a0caf944a35488c99722f8ab9947bf85bb6bdbc7)

spiral_matrix.php

$a = [
 [1, 2, 3,    4],
 [14, 15, 16, 5],
 [13, 20, 17, 6],
 [12, 19, 18, 7],
 [11, 10, 9,  8]
];

$r1 = rand(0, 7);
$r2 = rand(0, 7);
$a = [];
for ($i = 0; $i < $r1; $i++) {
 for ($j = 0; $j < $r2; $j++) {
   $a[$i][$j] = rand(1, 10);
}
}
$r = [];
for ($i = 0; $i < $r1; $i++) {
$t = [];
 for ($j = 0; $j < $r2; $j++) {
  $t[] = $a[$i][$j];
}
$r[] = '[' . implode(', ', $t) . ']';
}

echo "[\n" . implode(",\n", $r) . "\n];\n\n";

/**
some examples of array

$a = [
[6, 10, 9],
[3, 5, 2],
[8, 2, 5],
[4, 1, 8],
[6, 10, 5],
[6, 8, 10]
];


$a = [
 [1, 2, 3,    4],
 [14, 15, 16, 5],
 [13, 20, 17, 6],
 [12, 19, 18, 7],
 [11, 10, 9,  8]
];
*/

$rows = sizeof($a);
if ($rows) {
$cols = sizeof($a[0]);
} else {
 $cols = 0; 
}

// how much elements can be traversed
$total = $rows * $cols;

// limit, cols or rows
$i = $cols;

// mode, string 'rows' or 'cols' for toggle $i variable
$d = 'cols';

// counter of edge (cols edge or rows edge)
$j = 0;

// counter of items 
$c = 0;

// coordinates of cell ($y is row, $x is column)
$x = 0;
$y = 0;

// delta for coordinates
$dy = 0;
$dx = 1;

// resulting array
$r = [];

while ($c < $total) {
  // add data to result
  $r[] = $a[$y][$x];
  // increment counter of edge
  $j++;
  // if we finished traversing over current edge
  if ($j === $i) {
    // reset counter
    $j = 0;
    // swap direction deltas (1, 0) -> (0, 1) -> (-1, 0) -> (0, -1) -> (1, 0) -> ...
    list($dx, $dy) = [-$dy, $dx];
    // if it's last column in edge, decrement rows (because we are in some (last or first) row now, huh) and change mode
    if ($d === 'cols') {
       $rows--;
       $i = $rows;
       $d = 'rows';
    } else {
       // if it's last row in edge, decrement columns (because we are in some column now) and also change mode
       $cols--;
       $i = $cols;
       $d = 'cols';
    }
  }
  // change cell coordinates by delta
  $x = $x + $dx; $y = $y + $dy;

  // increment main counter
  $c++;
}

if (empty($r)) echo "Empty result\n";
echo implode(' ', $r);