spiral.php

<?php
class Spiral
{
    const RIGHT = 'RIGHT';
    const DOWN = 'DOWN';
    const LEFT = 'LEFT';
    const UP = 'UP';

    private $xMin = 0;
    private $xMax = 0;
    private $yMin = 0;
    private $yMax = 0;

    private $x = 0;
    private $y = 0;

    private $dirSequence = [self::RIGHT, self::DOWN, self::LEFT, self::UP];
    private $dir = 0;

    private $matrix;

    public function __construct(array $matrix)
    {
        $this->matrix = $matrix;
        $this->xMax = count($matrix[0]) - 1;
        $this->yMax = count($matrix) - 1;
    }

    public function walk()
    {
        while ($this->xMin <= $this->xMax && $this->yMin <= $this->yMax)
        {
            yield $this->currentItem();

            switch ($this->getDirection()) {
                case self::RIGHT:
                    if ($this->x == $this->xMax) {
                        $this->yMin++;
                        $this->dir++;
                        $this->y++;
                        continue;
                    }
                    $this->x++;
                    break;
                case self::LEFT:
                    if ($this->x == $this->xMin) {
                        $this->yMax--;
                        $this->dir++;
                        $this->y--;
                        continue;
                    }
                    $this->x--;
                    break;
                case self::UP:
                    if ($this->y == $this->yMin) {
                        $this->xMin++;
                        $this->dir++;
                        $this->x++;
                        continue;
                    }
                    $this->y--;
                    break;
                case self::DOWN:
                    if ($this->y == $this->yMax) {
                        $this->xMax--;
                        $this->dir++;
                        $this->x--;
                        continue;
                    }
                    $this->y++;
                    break;
            }
        }
        return;
    }

    private function getDirection()
    {
        return $this->dirSequence[$this->dir % 4];
    }

    private function currentItem()
    {
        return $this->matrix[$this->y][$this->x];
    }
}

$m = [
    [1, 2, 3],
    [8, 9, 4],
    [7, 6, 5],
];

$s = new Spiral($m);

foreach ($s->walk() as $value) {
    echo "$value\n";
}