为什么此PHP相对包含失败？

disc@puff:~/php$ ls 
a.php  data  include 

disc@puff:~/php$ tree 
. 
├── a.php 
├── data 
│   └── d.php 
└── include 
    ├── b.php 
    └── c.php 
2 directories, 4 files 

disc@puff:~/php$ cat a.php 
a.php is including include/b.php ... 
<?php include \"include/b.php\" ?> 

disc@puff:~/php$ cat include/b.php 
b.php is including c.php and ../data/d.php ... 
<?php include \"c.php\" ?> 
<?php include \"../data/d.php\" ?> 

disc@puff:~/php$ cat include/c.php 
c.php 

disc@puff:~/php$ cat data/d.php 
d.php 

disc@puff:~/php$ php a.php 
a.php is including include/b.php ... 
b.php is including c.php and ../data/d.php ... 
c.php 
PHP Warning:  include(../data/d.php): failed to open stream: No 
such file or directory in /home/disc/php/include/b.php on line 3 
PHP Warning:  include(): Failed opening \'../data/d.php\' for 
inclusion (include_path=\'.:/usr/share/php:/usr/share/pear\') in /home/ 
disc/php/include/b.php on line 3 

disc@puff:~/php$