public function rotate($angle) {
    // PHP 5.5 GD bug: https://bugs.php.net/bug.php?id=65148: To prevent buggy
    // behavior on negative multiples of 30 degrees we convert any negative
    // angle to a positive one between 0 and 360 degrees.
    $angle -= floor($angle / 360) * 360;

    // For some rotations that are multiple of 30 degrees, we need to correct
    // an imprecision between GD that uses C floats internally, and PHP that
    // uses C doubles. Also, for rotations that are not multiple of 90 degrees,
    // we need to introduce a correction factor of 0.5 to match the GD
    // algorithm used in PHP 5.5 (and above) to calculate the width and height
    // of the rotated image.
    if ((int) $angle == $angle && $angle % 90 == 0) {
      $imprecision = 0;
      $correction = 0;
    }
    else {
      $imprecision = -0.00001;
      $correction = 0.5;
    }

    // Do the trigonometry, applying imprecision fixes where needed.
    $rad = deg2rad($angle);
    $cos = cos($rad);
    $sin = sin($rad);
    $a = $this->width * $cos;
    $b = $this->height * $sin + $correction;
    $c = $this->width * $sin;
    $d = $this->height * $cos + $correction;
    if ((int) $angle == $angle && in_array($angle, [60, 150, 300])) {
      $a = $this->fixImprecision($a, $imprecision);
      $b = $this->fixImprecision($b, $imprecision);
      $c = $this->fixImprecision($c, $imprecision);
      $d = $this->fixImprecision($d, $imprecision);
    }

    // This is how GD on PHP5.5 calculates the new dimensions.
    $this->boundingWidth = abs((int) $a) + abs((int) $b);
    $this->boundingHeight = abs((int) $c) + abs((int) $d);

    return $this;
  }