matrix-snake.php

<?php
/**
  *
  * A snake matrix is a square matrix that follows this pattern:
  *
  * 3-by-3:

  * 1  2  3
  * 6  5  4
  * 7  8  9
  * and 4-by-4:
  * 
  * 1  2  3  4
  * 8  7  6  5
  * 9  10 11 12
  * 16 15 14 13
  */

<?php
$cols = 4;
$arr = range(1, 20);

(function() use ($cols, $arr){
    $chunks = array_chunk($arr, $cols);
    $counter = 1;
    $snakeMatrix = array_map(
        function($chunk) use (&$counter) {
            if (!($counter & 1)) {
                $counter++;
                return array_reverse($chunk);
            }
            
            $counter++;
            return $chunk;
        }, $chunks);

    foreach ($snakeMatrix as $numbers) {
        foreach ($numbers as $n)
            echo sprintf("%-5s", $n);
        echo PHP_EOL;
    }
})();

Output:

1    2    3    4    
8    7    6    5    
9    10   11   12   
16   15   14   13   
17   18   19   20