PHIGS was available as a standalone implementation (examples: Digital Equipment Corporation's DEC PHIGS, IBM's graPHIGS, Sun's SunPHIGS) and also used with the X Window system, supported via PEX, the "PHIGS Extension to X". PEX consisted of an extension to X, adding commands that would be forwarded from the X server to the PEX system for rendering. Workstations were placed in windows typically, but could also be forwarded to take over the whole screen, or to various printer-output devices.
PHIGS was designed in the 1980s, inheriting many of its ideas from the Graphical Kernel System of the late 1970s, and became an ANSI (ANSI X3.144-1988), FIPS (FIPS 153) and then ISO standard (ISO/IEC 9592 and ISO/IEC 9593 ) by 1989. Due to its early gestation, the standard supports only the most basic 3D graphics, including basic geometry and meshes, and only the basic Gouraud, "Dot", and Phong shading for rendering scenes. Features considered "standard" today, notably texture mapping, were not supported, nor were many machines of the era physically capable of it (at least in realtime).
The word "hierarchical" in the name refers to a notable feature of PHIGS: unlike most graphics systems, PHIGS included a scene graph system as a part of the basic standard. Models were built up in a Centralized Structure Store (CSS), a database containing a "world" including both the drawing primitives and their attributes (color, line style, etc.). CSSes could be shared among a number of views, known under PHIGS as a workstation.
Displaying graphics on the screen in PHIGS was a three-step process; first the model would be built into a CSS, then a workstation would be created and opened, and finally the model would be connected to the workstation. At that point the workstation would immediately render the model, and any future changes made to the model would instantly be reflected in the workstation view.
PHIGS originally lacked the capability to render illuminated scenes, and was superseded by PHIGS+. PHIGS+ works in essentially the same manner, but added methods for lighting a 3D scene, and was often referred to as PHIGS PLUS (where the PLUS was a slightly tongue-in-cheek acronym for "Plus Lumière Und Shading"). PHIGS+ also introduced more advanced graphics primitives, such as NURBS surfaces.
OpenGL, unlike PHIGS, is an immediate-mode rendering system with no "state"; once an object is sent to a view to be rendered it essentially disappears. Changes to the model have to be re-sent into the system and re-rendered, dramatically increasing programmer workload. For simple projects, PHIGS was considerably easier to use and work with.
However, OpenGL's "low-level" API allowed the programmer to make dramatic improvements in rendering performance by first examining the data on the CPU-side before trying to send it over the bus to the graphics engine. For instance, the programmer could "cull" the objects by examining which objects were actually visible in the scene, and sending only those objects that would actually end up on the screen. This was kept private in PHIGS, making it much more difficult to tune performance.
Given the low performance systems of the era and the need for high-performance rendering, OpenGL was generally considered to be much more "powerful" for 3D programming. PHIGS fell into disuse. Version 6.0 of the PEX protocol was designed to support other 3D programming models as well, but did not regain popularity. PEX was mostly removed from XFree86 4.2.x (2002) and finally removed from the X Window System altogether in X11R6.7.0 (April 2004)