PHP implicit pass by reference?

Question

I have this class which Resizes a given image to the set size and it can also put a watermark(stamp) on it.

  class Picture
    {

    private $src_path, $src_image, $width, $height, $resized_image;

    public function __construct($src_path)
    {
        if (!file_exists($src_path)) {
            throw new InvalidArgumentException('Invalid image path!');
        }
        $this->src_path = $src_path;

        $this->loadImage();
    }

    private function loadImage()
    {
        if (exif_imagetype($this->src_path) == IMAGETYPE_JPEG) {
            $this->src_image = imagecreatefromjpeg($this->src_path);
        } elseif (exif_imagetype($this->src_path) == IMAGETYPE_PNG) {
            $this->src_image = imagecreatefrompng($this->src_path);
        } else {
            throw new InvalidArgumentException('Your image type is not JPEG or PNG!');
        }
    }

    public function waterMark()
    {
        $stamp = imagecreatefromjpeg('C:\Users\Public\Pictures\Sample Pictures\resized.jpg');
        $im = $this->resized_image;

        $marge_right = 10;
        $marge_bottom = 80;
        $sx = imagesx($stamp);
        $sy = imagesy($stamp);

        // Copy the stamp image onto our photo using the margin offsets and the photo 
        // width to calculate positioning of the stamp. 
        imagecopy($im, $stamp, imagesx($im) - $sx - $marge_right, imagesy($im) - $sy - $marge_bottom, 0, 0, imagesx($stamp), imagesy($stamp));
    }

    public function resizeImage($w, $h)
    {
        list($width, $height) = getimagesize($this->src_path);
        $ratio = $width / $height;

        if ($w / $h > $ratio) {
            $new_width = $h * $ratio;
            $new_height = $h;
        }else {
            $new_height = $w / $ratio;
            $new_width = $w;
        }

        $this->resized_image = imagecreatetruecolor($new_width, $new_height);

        imagecopyresampled($this->resized_image, $this->src_image, 0, 0, 0, 0, $new_width, $new_height, $width, $height);
    }

    public function saveImage($quality, $destination = null)
    {
        if (!$destination) {
            $length = strlen($this->src_path) - strlen((string) $this->src_image);

            $destination = substr($this->src_path, 0, $length-3).'resized.jpg';
        }

        return imagejpeg($this->resized_image, $destination, $quality);
    }
}

$pic = new Picture('C:/Users/Public/Pictures/Sample Pictures/Chrysanthemum.jpg');

$pic->resizeImage(400, 400);
$pic->saveImage(100, 'C:/Users/Public/Pictures/ResizedImage.jpg');
$pic->waterMark();

//Why is the original picture ($this->resized_image)
//is now watermarked ?
//Why is there a reference here ?
$pic->saveImage(100, 'C:/Users/Public/Pictures/WhyAmIWaterMarked.jpg');

My question is after I use the WaterMark() function why is the $this->resized_image variable changes to the value of the WaterMarked one ?

$im = $this->resized_image;
imagecopy($im, $stamp, imagesx($im) - $sx - $marge_right, imagesy($im) - $sy - $marge_bottom, 0, 0, imagesx($stamp), imagesy($stamp));

The result:

I wrote another class just to test this behaviour:

class TestReference
{
    public $foo;

    function setFoo($value)
    {
        $this->foo = $value;
    }

    function getFoo()
    {
        return $this->foo;
    }

    function getFooBar()
    {
        $tmp = $this->foo;
        $tmp = 20;

        return $this->foo;
    }
}

$bar = new TestReference();
$bar->setFoo(15);
echo $bar->getFoo().'<br/>';
echo $bar->getFooBar().'<br/>';
echo $bar->getFoo().'<br/>';

But the result is as expected: 15, 15, 15;

So how come in the Picture class that variable($this->resized_image) is overrwriten by a temporary variable ?