A need that came up today, was that this particular layer, being served from ArcGIS REST, is polygons and we need points. Normally I would calculate the centroid and use that... but this specific software is being developed on a plain web host: no GEOS or OGR, no PostGIS... just plain ol' PHP.
Some Reading... then Writing
So I did some reading:
And wrote this, a pure PHP function for calculating the signed area of a polygon, and then the centroid of the polygon:
As described in the file, this is specific to our sorts of use cases: park boundaries, city boundaries, and the like. If the polygon is self-intersecting (that's a no-no) it may be wrong. If it's an L shape it may come up wrong too. And if it's a multipolygon and those rings overlap, it'll double-count the area, as it's not smart enough to find intersecting area between two rings and subtract it out (and this incorrect area may affect the centroid placement).
But, it does what we need it to do, and it may work for you. Enjoy!