Task: Maze generation
C#: using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Drawing;

namespace MazeGeneration
{
    public static class Extensions
    {
        public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> source, Random rng)
        {
            var e = source.ToArray();
            for (var i = e.Length - 1; i >= 0; i--)
            {
                var swapIndex = rng.Next(i + 1);
                yield return e[swapIndex];
                e[swapIndex] = e[i];
            }
        }

        public static CellState OppositeWall(this CellState orig)
        {
            return (CellState)(((int) orig >> 2) | ((int) orig << 2)) & CellState.Initial;
        }

        public static bool HasFlag(this CellState cs,CellState flag)
        {
            return ((int)cs & (int)flag) != 0;
        }
    }

    [Flags]
    public enum CellState
    {
        Top = 1,
        Right = 2,
        Bottom = 4,
        Left = 8,
        Visited = 128,
        Initial = Top | Right | Bottom | Left,
    }

    public struct RemoveWallAction
    {
        public Point Neighbour;
        public CellState Wall;
    }

    public class Maze
    {
        private readonly CellState[,] _cells;
        private readonly int _width;
        private readonly int _height;
        private readonly Random _rng;

        public Maze(int width, int height)
        {
            _width = width;
            _height = height;
            _cells = new CellState[width, height];
            for(var x=0; x<width; x++)
                for(var y=0; y<height; y++)
                    _cells[x, y] = CellState.Initial;
            _rng = new Random();
            VisitCell(_rng.Next(width), _rng.Next(height));
        }

        public CellState this[int x, int y]
        {
            get { return _cells[x,y]; }
            set { _cells[x,y] = value; }
        }

        public IEnumerable<RemoveWallAction> GetNeighbours(Point p)
        {
            if (p.X > 0) yield return new RemoveWallAction {Neighbour = new Point(p.X - 1, p.Y), Wall = CellState.Left};
            if (p.Y > 0) yield return new RemoveWallAction {Neighbour = new Point(p.X, p.Y - 1), Wall = CellState.Top};
            if (p.X < _width-1) yield return new RemoveWallAction {Neighbour = new Point(p.X + 1, p.Y), Wall = CellState.Right};
            if (p.Y < _height-1) yield return new RemoveWallAction {Neighbour = new Point(p.X, p.Y + 1), Wall = CellState.Bottom};
        }

        public void VisitCell(int x, int y)
        {
            this[x,y] |= CellState.Visited;
            foreach (var p in GetNeighbours(new Point(x, y)).Shuffle(_rng).Where(z => !(this[z.Neighbour.X, z.Neighbour.Y].HasFlag(CellState.Visited))))
            {
                this[x, y] -= p.Wall;
                this[p.Neighbour.X, p.Neighbour.Y] -= p.Wall.OppositeWall();
                VisitCell(p.Neighbour.X, p.Neighbour.Y);
            }
        }

        public void Display()
        {
            var firstLine = string.Empty;
            for (var y = 0; y < _height; y++)
            {
                var sbTop = new StringBuilder();
                var sbMid = new StringBuilder();
                for (var x = 0; x < _width; x++)
                {
                    sbTop.Append(this[x, y].HasFlag(CellState.Top) ? "+---" : "+   ");
                    sbMid.Append(this[x, y].HasFlag(CellState.Left) ? "|   " : "    ");
                }
                if (firstLine == string.Empty)
                    firstLine = "   " + sbTop.ToString();
                Debug.WriteLine("   " + sbTop + "+");
                Debug.WriteLine("   " + sbMid + "|");
            }
            Debug.WriteLine(firstLine);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var maze = new Maze(20, 20);
            maze.Display();
        }
    }
}

PHP: <?php
class Maze 
{
    protected $width;
    protected $height;
    protected $grid;
    protected $path;
    protected $horWalls;
    protected $vertWalls;
    protected $dirs;
    protected $isDebug;

    public function __construct($x, $y, $debug = false)
    {
        $this->width = $x;
        $this->height = $y;
        $this->path = [];        
        $this->dirs = [ [0, -1], [0, 1], [-1, 0], [1, 0]]; // array of coordinates of N,S,W,E
        $this->horWalls = []; // list of removed horizontal walls (---+)        
        $this->vertWalls = [];// list of removed vertical walls (|)        
        $this->isDebug = $debug; // debug flag

        $this->generate();
    }

    protected function generate()
    {        
        $this->initMaze(); // init the stack and an unvisited grid

        $this->walk(mt_rand(0, $this->width-1), mt_rand(0, $this->height-1));
    }

    
    public function printOut()
    {
        $this->log("Horizontal walls: %s", json_encode($this->horWalls));
        $this->log("Vertical walls: %s", json_encode($this->vertWalls));        
        
        $northDoor = mt_rand(0,$this->width-1);
        $eastDoor = mt_rand(0, $this->height-1);

        $str = '+';
        for ($i=0;$i<$this->width;$i++) {
            $str .= ($northDoor == $i) ? '   +' : '---+';
        }
        $str .= PHP_EOL;
        for ($i=0; $i<$this->height; $i++) {
            
            for ($j=0; $j<$this->width; $j++) {
                $str .= (!empty($this->vertWalls[$j][$i]) ? $this->vertWalls[$j][$i] : '|   ');
            }
            $str .= ($i == $eastDoor ? '  ' : '|').PHP_EOL.'+';
            for ($j=0; $j<$this->width; $j++) {
                $str .= (!empty($this->horWalls[$j][$i]) ? $this->horWalls[$j][$i] : '---+');
            }
            $str .= PHP_EOL;
        }
        echo $str;
    }

    
    protected function log(...$params)
    {
        if ($this->isDebug) {
            echo vsprintf(array_shift($params), $params).PHP_EOL;
        }
    }

    private function walk($x, $y)
    {
        $this->log('Entering cell %d,%d', $x, $y);

        $this->grid[$x][$y] = true; 

        $this->path[] = [$x, $y];

        $neighbors = $this->getNeighbors($x, $y);       
        $this->log("Valid neighbors: %s", json_encode($neighbors));

        if(empty($neighbors)) {

            $this->log("Start backtracking along path: %s", json_encode($this->path));
            array_pop($this->path);
            if (!empty($this->path)) {
                $next = array_pop($this->path);
                return $this->walk($next[0], $next[1]);
            }
        } else {            

            shuffle($neighbors);

            foreach ($neighbors as $n) {
                $nextX = $n[0];
                $nextY = $n[1];
                if ($nextX == $x) {
                    $wallY = max($nextY, $y);
                    $this->log("New cell is on the same column (%d,%d), removing %d, (%d-1) horizontal wall", $nextX, $nextY, $x, $wallY);
                    $this->horWalls[$x][min($nextY, $y)] = "   +";
                }
                if ($nextY == $y) {
                    $wallX = max($nextX, $x);
                    $this->log("New cell is on the same row (%d,%d), removing %d,%d vertical wall", $nextX, $nextY, $wallX, $y);
                    $this->vertWalls[$wallX][$y] = "    ";              
                }
                return $this->walk($nextX, $nextY);
            }
        }
    }
    
    
    private function initMaze()
    {
        for ($i=0;$i<$this->width;$i++) {
            for ($j = 0;$j<$this->height;$j++) {
                $this->grid[$i][$j] = false;
            }
        }
    }

    
    private function getNeighbors($x, $y) 
    {       
        $neighbors = [];
        foreach ($this->dirs as $dir) {
            $nextX = $dir[0] + $x;
            $nextY = $dir[1] + $y;
            if (($nextX >= 0 && $nextX < $this->width && $nextY >= 0 && $nextY < $this->height) && !$this->grid[$nextX][$nextY]) {
                $neighbors[] = [$nextX, $nextY];
            }
        }
        return $neighbors;
    }
}

$maze = new Maze(10,10);
$maze->printOut();