Phing was originally a subproject of Binarycloud. Binarycloud is a highly engineered application framework, designed for use in enterprise environments. Binarycloud uses XML extensively for storing metadata about a project (configuration, nodes, widgets, site structure, etc.). Because Binarycloud is built for PHP, performing extensive XML processing and transformations on each page request is an unrealistic proposition. Phing is used to "compile" the XML metadata into PHP arrays that can be processed without overhead by PHP scripts.
Of course, XML compilation is only one of many ways that Binarycloud uses the Phing build system. The Phing build system makes it possible for you to:
Build multi language pages from one source tree,
Centralize metadata (e.g. your data model) in one XML file and generate several files from that XML with different XSLT.
It the beginning, Binarycloud used the GNU make system; however, this approach had some drawbacks: The space-before-tab-problem in makefiles, the fact that it is only natively available for Unix systems etc. So, the need for a better build system arose. Due to its XML build files and modular design, Apache Ant was a logical choice. The problem was that Ant is written in Java, so you need to install a JVM on your computer to use it. Besides the need for yet another interpreter (i.e. besides PHP), there was also legal/ideological conflict in requiring a commercial JVM (there were problems with Ant on JVMs other than Sun's) for an LGPL'd Binarycloud.
So, the development of Phing began. Phing is a build system written in PHP and uses the ideas of Ant. The first release was designed & developed simultaneously, and thus not very sophisticated. This original system was quickly pushed to its limits and the need for a better Phing became a priority. Andreas Aderhold, who was responsible for Phing/r1, designed and wrote much of the Phing/r2 that followed. Phing/r2 became the Phing-1.0 that exists today for PHP4.
The current development version of Phing 2.x, requires PHP5 (at least 5.2.x) and makes use of many of the available features in PHP5.2 to achieve a high degree of modularization, code efficiency as well as stability and testability.