Predict is a partial PHP port of the Gpredict program that allows real-time tracking and orbit prediction of satellites from two line element sets. It supports the SGP4 and SDP4 models for prediction.
Just clone this repo and try out the tests and examples from the root of the checkout.
The tests directory includes a port of the original sgpsdp test files from Gpredict. They are pretty close.
Included in the examples directory is a sample iss.tle (with an update script, which you should run first). There are two examples, the visible_passes.php script and the benchmark.php script. The former is for generating visible pass predictions of the ISS, and its output is similar to what you might get from the Heavens-Above website, and it is heavily commented. The latter just does predictions for benchmarking with xhprof.
You can see an image of a Predict/Google Maps API mash-up I did for fun:
You can also see an example visible pass plotted from the polar view of the observer:
About this port
This port largely maintains the style and organization of the original C code, but scopes methods into classes rather than leaving everything in the global scope. The motivation for this is so that changes upstream can more easily be integrated over time. Only the prediction routines have been ported.