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    <p>In my experience you can best solve this by uploading your files to a directory that is not available publicly. Then you would also have some table in your database where you'd have at least three columns: the <em>name</em> of the file, the <em>access level</em> you want to give it and the <em>user</em> that uploaded the file. </p> <p>You could then write something that checks if the visiting user has access to the file. First it would retrieve the column containing information about the mentioned file. Based on that information, if the user has access, use <a href="http://php.net/readfile" rel="nofollow">readfile</a> (see the example on the PHP manual) to display the file. In all other case you could just show a 403 Forbidden page.</p> <p>In your case the function that checks access would look something like this:</p> <pre><code>function hasAccess($accesslevel, $owner, $visitor) { if ($accesslevel === 'public') { return true; } if ($accesslevel === 'private') { return $owner === $visitor; } if ($accesslevel === 'friends') { return $owner === $visitor || isfriend($visitor, $owner); } } </code></pre> <p>This is just an example, your implementation would probably be a little different, depending on your environment.</p> <p>As far as I know Facebook just allows all files to be viewed by everyone, but tries to create a url that is difficult to guess. As soon as you've got the url, you can just view the image itself, regardless of the privacy-settings of the user that uploaded the image.</p>
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    1. COI wasn't aware that facebook allows seeing an image even while it is private. Anyway, well facebook has around 800M accounts. If every account has at least 100 pictures, that becomes 80 billion files. Either they use a different technique or same technique with thousands of servers.
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    2. CO@Johnny Actually, facebook uses a naming scheme like `[user_id]_[photograph_id]_[random_number]_size` (not sure about the exact scheme), they will probably use a 64bit photograph id, which would allow about 10 to the power of 20 photo's per user. It's safe to say no person would ever reach that... ever! 80 billion is just a tiny tiny tiny fraction of a fraction of the largest number allowed in a 64bit number.
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