Conway - game of life

<?php
class Conway {
    private $population = array(
        array(1,0,1),
        array(1,1,1),
        array(0,1,0)
    );
    private $nextPopulation = array();
    private $time=0;

    /* You can set a specific population from the outside */
    public function setPopulation($arr) {
        $this->population = $arr;
        $this->time=0;
    }

    /* Calculate the next generation */
    public function nextGen() {
        for($x = 0; $x < count($this->population); $x++) {
            for($y = 0; $y < count($this->population[$x]); $y++) {
                $nbs = $this->_countNeighboursAlive($x,$y);
                if($nbs == 3) $this->nextPopulation[$x][$y] = 1;
                elseif($nbs < 2 or $nbs > 3) $this->nextPopulation[$x][$y] = 0;
                else $this->nextPopulation[$x][$y] = $this->population[$x][$y];
            }
        }
        $this->population = $this->nextPopulation;
        $this->time++;
    }

    /* count all existent and alive neighbours */
    private function _countNeighboursAlive($x,$y) {
        $neighbours = $this->_getNeighbours($x,$y);
        $ret = 0;
        foreach($neighbours as $n) {
            if(isset($this->population[$n[0]][$n[1]]) and $this->population[$n[0]][$n[1]]) $ret++;
        }
        return $ret;
    }

    /* It returns all possibilities of neighbours, despite the fact, some might not exists */
    private function _getNeighbours($x,$y) {
        $neighbours = array();
        array_push($neighbours, array($x-1,$y-1));
        array_push($neighbours, array($x-1,$y));
        array_push($neighbours, array($x-1,$y+1));

        array_push($neighbours, array($x,$y-1));
        array_push($neighbours, array($x,$y+1));

        array_push($neighbours, array($x+1,$y-1));
        array_push($neighbours, array($x+1,$y));
        array_push($neighbours, array($x+1,$y+1));

        return $neighbours;
    }

    public function get() {
        for($x = 0; $x < count($this->population); $x++) {
            for($y = 0; $y < count($this->population[$x]); $y++) {
                print ($this->population[$x][$y] ? ' # ' : ' . ');
            }
            print "\n";
        }
        print "\nGeneraten: ".$this->time;
    }
}
?>