I’ve done a bunch of GIS development in my past programming days. Almost all of my experience was with the openSource GIS engine MapServer. MapServer is mainly a set of cgi-bin programs that let you read and display various map data such as ESRI Shape files, TIFF & geoTifff, and even ESRI ArchSDE data stored on remote servers. There is a scripting subset of MapServer called MapScript which lets you integrate GIS features into your PHP, Microsoft .NET, Perl, Python and Java based applications.

Around 2003 I was able to make a fully function PHP based GIS system which loaded ESRI Shape files, displayed them as a jpeg on a website and let users zoom in and out, and even select polygons for displaying the underlying geoData. Eventually I was even able to add a geoTIFF layer to display aerial photos.

I highly recommend this free GIS engine to anyone that is interest in integrating GIS features into their web based or even desktop based applications.

Features:
Support for popular scripting and development environments
PHP, Python, Perl, Ruby, Java, and .NET

Cross-platform support
Linux, Windows, Mac OS X, Solaris, and more

Support of numerous Open Geospatial Consortium (OGC) standards
WMS (client/server), non-transactional WFS (client/server), WMC, WCS, Filter Encoding, SLD, GML, SOS, OM

A multitude of raster and vector data formats
TIFF/GeoTIFF, EPPL7, and many others via GDAL
ESRI shapfiles, PostGIS, ESRI ArcSDE, Oracle Spatial, MySQL and many others via OGR