html2canvas php proxy

html2canvas-proxy.php

<?php

/**
 * html2canvas proxy script
 *
 * Provides proxy service for html2canvas.js lib.
 * Responsible for output javascript callback with image url
 * to provide work around for the same origin limitation.
 * 
 * Used to ensure that project images are included in the export PNGs,
 * as a client side JS request for the file will trip up on CORS issues.
 *
 * This script is locked down to certain origins and works only with images.
 * 
 * The origin of the request should come from either:
 *   - *.typecast.com
 *   - *.frontinternal.net (our dev vhosts)
 *
 * Only images are allowed to be proxied through the script.
 * 
 * Images are detected by content type header.
 *
 * @author Eugene Glova
 */
class Html2CanvasProxy {

    // Reference to the url
    private $url;

    // Reference to the callback name
    private $callback_name;

    // List of the allowed origins
    private $allowed_origins = array(
        'typecast.com',
        'frontinternal.net'
    );

    /**
     * __construct
     *
     * Constructor. Accepts url and callback name
     *
     * @param  string $url
     * @param  string $callback_name
     * @return Html2CanvasProxy
     */
    public function __construct($url, $callback_name) {
        // Check url
        if ( ! is_string($url) || strlen($url) < 1) return false;

        // Check callback_name
        if ( ! is_string($callback_name) || strlen($callback_name) < 1) return false;

        // Check for origin
        if ( ! $this->isAllowedOrigin()) return false;

        // Set url
        $this->url = $url;

        // Set callback name
        $this->callback_name = $callback_name;

        return $this;
    }


    /**
     * getContent
     *
     * Returns javascript callback with argument
     *
     * @return string
     */
    public function getContent() {
        // Set http headers
        $this->setHeaders();

        // Get callback with content
        return $this->getCallback();
    }


    /**
     * isAllowedOrigin
     *
     * Returns true if the http referer is in list of allowed origins
     *
     * @return boolean
     */
    private function isAllowedOrigin() {
        // Get http referer host
        $host = parse_url($_SERVER['HTTP_REFERER'], PHP_URL_HOST);

        // Loop through allowed origins
        foreach ($this->allowed_origins as $allowed_host) {
            // Compare referer host with allowed origins
            if (strstr($host, $allowed_host) === $allowed_host) return true;
        }

        return false;
    }


    /**
     * setHeaders
     *
     * Sets application/javascript content type http header
     *
     * @return boolean
     */
    private function setHeaders() {
        // Set content type header for the output
        header('Content-Type: application/javascript');

        return true;
    }


    /**
     * getImageBase64DataUrl
     *
     * Downloads image data by provided url
     * and returns base64 encoded data url like
     * data:image/png;base64,...
     * 
     * @param  string $url
     * @return string
     */
    private function getImageBase64DataUrl() {
        // Get url scheme
        $scheme = parse_url($this->url, PHP_URL_SCHEME);

        // Check url scheme
        if (strpos($scheme, 'http') !== 0) return false;

        // Get data by url
        $data = file_get_contents($this->url);

        // Check data
        if ( ! is_string($data) || strlen($data) < 1) return false;

        // Get content type
        foreach ($http_response_header as $header) {
            // Check for image content type
            if ( ! $this->isImageContentTypeHeader($header)) continue;

            // Get content type
            $content_type = $this->getContentTypeByHeader($header);

            break;
        }

        // Check content type
        if ( ! is_string($content_type) || strlen($content_type) < 1) return false;

        return 'data:' . $content_type . ';base64,' . base64_encode($data);
    }


    /**
     * isImageContentTypeHeader
     *
     * Returns true if 'Content-type' header is like 'image/jpeg'
     *
     * @param  string  $header
     * @return boolean
     */
    private function isImageContentTypeHeader($header) {
        // Check header
        if ( ! is_string($header) || strlen($header) < 1) return false;

        // Check content type for image
        if ( ! preg_match('/^content-type:\s*image\/\w+/i', $header)) return false;

        return true;
    }


    /**
     * getContentTypeByHeader
     *
     * Returns сontent type like 'image/jpeg'
     *
     * @param  string $header
     * @return string
     */
    private function getContentTypeByHeader($header) {
        // Check header
        if ( ! is_string($header) || strlen($header) < 1) return false;

        // Check for type and get it
        if ( ! preg_match('/^content-type:\s*(\w+\/\w+)/i', $header, $matches)) return false;

        return $matches[1];
    }


    /**
     * getCallback
     *
     * Returns javascript callback
     * 
     * @return string
     */
    private function getCallback() {
        return $this->callback_name . '(' . json_encode($this->getImageBase64DataUrl()) . ')';
    }

}

// Create an instance of the class
$proxy = new Html2CanvasProxy($_GET['url'], $_GET['callback']);

// Output content
echo $proxy->getContent();