lib/PHPExif/Exif.php
public function getHorizontalResolution()
    {
        if (!isset($this->data[self::HORIZONTAL_RESOLUTION])) {
            return false;
        }

        return $this->data[self::HORIZONTAL_RESOLUTION];
    }
public function getHorizontalResolution()
    {
        if (!isset($this->data[self::HORIZONTAL_RESOLUTION])) {
            return false;
        }

        return $this->data[self::HORIZONTAL_RESOLUTION];
    }
Returns the horizontal resolution in DPI, if it exists

@return int|boolean