PHP provides a large number of predefined constants to
any script which it runs. Many of these constants, however,
are created by various extensions, and will only be present
when those extensions are available, either via dynamic
loading or because they have been compiled in.
There are four magical constants that change depending
on where they're used. For example, the value of __LINE__ depends on the line that it's
used on in your script. These special constants are
case-insensitive and are as follows:
表格 9-1. A few "magical" PHP "constants"
A list of predefined constants is available in the
section Reserved predefined
constants.