# 6.5 Build Lifecycle This section exists to explain -- or try -- how Phing "works". Particularly, how Phing proceeds through a build file and invokes tasks and types based on the tags that it encounters. 6.5.1 How Phing Parses Buildfiles Phing uses an ExpatParser class and PHP's native expat XML functions to handle the parsing of build files. The handler classes all extend the phing.parser.AbstractHandler class. These handler classes "handle" the tags that are found in the buildfile. Core tasks and datatypes are mapped to XML tag names in the defaults.properties files -- specifically phing/tasks/defaults.properties and phing/types/defaults.properties. It works roughly like this: 1. `phing.parser.RootHandler` is registered to handle the buildfile XML document 2. RootHanlder expects to find exactly one element: `<project>`. RootHandler invokes the ProjectHandler with the attributes from the <project> tag or throws an exception if no <project> is found, or if something else is found instead. 3. `ProjectHandler` expects to find `<target>` tags; for these `ProjectHandler` invokes the `TargetHandler`. ProjectHandler also has exceptions for handling certain tasks that can be performed at the top-level: `<resolve>`, `<taskdef>`, `<typedef>`, and `<property>`; for these `ProjectHandler` invokes the TaskHandler class. If a tag is presented that doesn't match any expected tags, then `ProjectHandler` assumes it is a datatype and invokes the `DataTypeHandler`. 4. `TargetHandler` expects all tags to be either tasks or datatypes and invokes the appropriate handler (based on the mappings provided in the `defaults.properties` files). 5. Tasks and datatypes can have nested elements, but only if they correspond to a create\*() method in the task or datatype class. E.g. a nested `<param>` tag must correspond to a `createParam()` method of the task or datatype. `... More to come ...`