PHPCoord can convert coordinates representing a point from one Coordinate Reference System (CRS) to another one. In this simplest case this might just be converting units (e.g. feet to metres or vice versa), but in most cases involves running one or more complex algorithms on the numbers.
Most such algorithms are not standalone, but require parameters to work - for instance many require an origin point to be specified and these points differ across mapping systems even when they utilise the same algorithm. Knowing both the algorithm(s) to be used and the parameters used to tune them is essential to perform high-accuracy conversions.
If the Earth were actually the shape of an ellipsoid, algorithms could be devised so that conversions between systems could be performed with no absolutely no loss of accuracy - systems would in effect be mathematically equivalent.
Unfortunately the Earth isn’t an ellipsoid and coordinates are determined by individual humans operating on the Earth’s actual, irregular surface using instruments subject to observation error. That means that conversions between CRSs are not just converting between mathematical ideals but often convert between sets of observations. When this happens it means that the conversions between the CRSs can only ever be an approximation (typically within a few metres) rather a precise transformation.
A corollary is that when dealing with CRSs that cover significant land area it is possible (and common) for mapping agencies to derive multiple different parameter sets for use to obtain better accuracy depending on location. For example when converting from ED50 to ETRS89 you would ideally use slightly different parameters for a point inside Portugal than for a point inside Norway.
PHPCoord offers two models of operation for coordinate conversion:
The hard way in which you can directly utilise one of the many implemented algorithms along with the parameters of your choice to keep full control over the conversion
The easy way in which PHPCoord consults the built-in EPSG dataset for the relevant algorithm(s) to use and best parameters to use for your chosen conversion and executes them behind the scenes on your behalf.