多数情况下，两种方式的区别可能在性能上有细微的区别(via)。 但是，在多重包含的情况下表现未必一样：
|-- index.php
`-- lib
|-- a.php
`-- b.php假定有三个文件，入口文件index.php，包含lib/a.php。a.php又需要包含它同目录下的b.php：
我们通过strace来跟踪一下方式2的系统调用(我的环境下include_path=.:/tmp)：
open("/home/robin/devel/lib/a.php", O_RDONLY) = 3
open("/home/robin/devel/b.php", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/tmp/b.php", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/home/robin/devel/lib/b.php", O_RDONLY) = 3官方文档的阐述是这样的：
Files are included based on the file path given or, if none is given, the include_path specified. If the file isn’t found in the include_path, include() will finally check in the calling script’s own directory and the current working directory before failing.
所以优先级应该是这样：
当然，比较推荐结合魔术常量__FILE__来include。