You probably have to distinguish here between URLs and file paths.
When you direct your browser to a an image like
http://www.example.com/path/to/image.jpg
and it works, then still the file_exists()
function will return false for this URL, because it's not an image path.
The correct path would be something like
/var/www/htdocs/path/to/image.jpg
on the local file system. file_exists()
would then return true for this path.
What you need to test for with file_exists()
is the local path of the image. If it exists, then you need to include the URL of the image. You do the URL inclusion correctly, but not the path usage.