This is totally possible you can write for example a JAVA/VS program on the server that takes some console parameters for the texture file you wish to use and where the user uploaded the texture.
You would have a PHP script call this program and give it the right parameters via the PHP shell commands. When it executes screen renders will be taken and saved to a directory accessible by PHP.
Then simply show them on the page.
This is all theoretical however and much easier ways of doing this can be used. Many of those ways no server side processing is required for example a flash applet/java applet can do this far easier and is all client side so you won't need a powerful server to do the rendering.
Please keep in mind most servers do not have a graphics renderer or one that is powerful enough to do this at any scale beyond just a few users. Also it is important to remember that PHP is server side and won't be able to produce a real time 3d application like a flash applet or java applet would.
On another note you could do all of this in HTML5, but it is a lot of work and the technology required is not in most browsers and the ones it is are touch and go.
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