GOL php simple implementation

Cell.php

<?php

class Cell {

    public $x;
    public $y;
    
    public $surroundings = [];
    
    public function __construct($x, $y)
    {
        $this->x = $x;
        $this->y = $y;
        array_push($this->surroundings, ($x) . "|" . ($y+1));
        array_push($this->surroundings, ($x) . "|" . ($y-1));
        array_push($this->surroundings, ($x+1) . "|" . ($y));
        array_push($this->surroundings, ($x-1) . "|" . ($y));
        array_push($this->surroundings, ($x-1) . "|" . ($y-1));
        array_push($this->surroundings, ($x+1) . "|" . ($y-1));
        array_push($this->surroundings, ($x-1) . "|" . ($y+1));
        array_push($this->surroundings, ($x+1) . "|" . ($y+1));
        
    }

}

GolTest.php

<?php


class GolTest extends PHPUnit_Framework_TestCase {

    /**
     * @test
     */
    public function shouldDieIfHaveLessThan2Neighbours(){

        $world = new World(
            array(
                "1|1" => new Cell(1,1)
            )
        );
        $world->step();
        $this->assertEquals(false, isset($world->cells["1|1"]));
    }

  
}

World.php

<?php

class World {
    
    public $cells;
    
    public function __construct($cells)
    {
        $this->cells = $cells;
    }

    public function step(){
        $newWorld = [];
        foreach ($this->cells as $cell) {
            $siblingsCount = 0;
            foreach($cell->surroundings as $surrounding) {
                if(isset($this->cells[$surrounding])) {
                    $siblingsCount++;
                } else {
                    $surroundingArray = preg_split('/\|+/',$surrounding);
                    $tmpCell = new Cell($surroundingArray[0],$surroundingArray[1]);
                    $tmpSiblingCount = 0;
                    foreach ($tmpCell->surroundings as $tmpSurrounding) {
                        if(isset($this->cells[$tmpSurrounding])) {
                            $tmpSiblingCount++;
                        }
                    }
                    if($tmpSiblingCount == 3) {
                        $newWorld[$tmpCell->x . "|" . $tmpCell->y] = $tmpCell;
                    }
                }
            }
            if($siblingsCount == 2 || $siblingsCount == 3) {
                $newWorld[$cell->x . "|" . $cell->y] = $cell;
            }
            
        }
        $this->cells = $newWorld;
    }
    
}