Absolute path does not work

[hvle]

I've encounted a strange behaviour:

my script is in this path:
/home/user/public_html/admin/admin_func.php
which change mod a file name /home/user/public_html/au/adelaide/index_en.html to 0777

when I used a relative path like below:
ftp_site($ftp_conn, "CHMOD 0777 ../au/adelaide/index_en.html");
this works fine.


however if I have:
ftp_site($ftp_conn, "CHMOD 0777 /home/user/public_html/au/adelaide/index_en.html");
I got the error below
----------------
Warning: ftp_site(): Could not change perms on /home/user/public_html/au/adelaide/index_en.html: No such file or directory in /home/user/public_html/admin/admin_func.php on line 12
----------------

Please help!!!!