PHP - 检查图像是否加载到 img src 中或直接在浏览器中加载？

Question

Suppose i have an image url :- example.com/myimg.png

I want to check whether it is included inside img src or someone directly opened it via browser.

What i need:- Check this image of GIPHY - https://media3.giphy.com/media/N8Lfh9gWcWYIU/giphy.gif

if you directly open it inside browser then it contains links at top and bottom of image but if you include this image inside img src then it only displays an image.

I tried below code but it is not a reliable solution:-

$ref = isset($_SERVER['HTTP_REFERER'])? $_SERVER['HTTP_REFERER']: "";
if ($ref != "")
{
    header( 'Content-Type: image/jpeg' );
    readfile( 'my-image.jpg' );
}
else
{
  <img src="my-image.jpg">
  <a href="#">My Link</a>
}

Answer 1

TL;DR giphy analyzes Accept HTTP header and also uses Age: header value greater than Cache-Control: max-age= value

When your browser loads an image in <img> tag, it sends specific Accept header. Firefox 71.0b12, for example, will use

Accept: image/webp,*/*

But if you open this URL in browser, even with same Referer header, the browser will send another request with header

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8

But there is more. Due to caching browser will not send another request.

To avoid this, giphy will reply with following headers:

Last-Modified: ...
Etag: "..."
Content-Type: image/gif
Cache-Control: max-age=86400
Age: 99360

(other headers omitted, order not preserved)

As you can see, Age is greater than max-age , so it is considered "stale" by the browser instantly , forcing another request when opening image directly.

Note that this method may not work in all browsers.