Linux (and UNIX, BSD, Solaris) are all based on the idea that a file's owner, group, and everyone else have different access needs. The simple overview is that there are different permissions for reading a file, writing a file, and executing a file; and there are three user types with each of these three permissions.

The file's owner can read, write and/or execute the file.
Members of the file's group can read, write and/or execute the file. 
Everyone else can read, write and/or execute the file.

Bring up a terminal shell on your desktop. Typically this will be a right-click on the desktop labeled something like "Open Terminal". When you see the prompt, type ls to get a simple directory listing.

[bob@linuxhost ~]$ ls
Desktop  Documents  Downloads  file.txt

[bob@linuxhost ~]$ ls -a
. .. .bash_history .bash_logout .bash_profile .bashrc Desktop  Documents  Downloads  file.txt

Let's expand a little. Let's look at a 'long' directory list. Type ls -al. You should see something like:

[bob@linuxhost ~]$ ls -al
total 16
drwxr-xr-x 2 bob bob   4096 Jan 21  2009 .
drwxr-xr-x 8 root root 4096 Jul  1 23:31 ..
-rw------- 4 bob bob   2189 Oct 21 10:15 .bash_history
-rw-r--r-- 4 bob bob     18 Oct 21 10:15 .bash_logout
-rw-r--r-- 4 bob bob    141 Oct 21 10:15 .bash_profile
-rw-r--r-- 4 bob bob    492 Oct 21 10:15 .bashrc
drwxr-xr-x 4 bob bob   4096 Oct 21 10:15 Desktop
drwxr-xr-x 4 bob bob   4096 Oct 21 10:15 Documents
drwxr-xr-x 4 bob bob   4096 Oct 21 10:15 Downloads
-rwxrwxrwx 4 bob bob    256 Oct 21 10:15 file.txt

So who owns the file or directory? The 3rd and 4th fields both say 'root'. The 3rd field is the owner's name, and the 4th field is the name of the group.

-rwxrwxrwx 4 root   root  256 Oct 21 10:15 file.txt
             ^owner ^group

So who am I and what group am I associated with? Sounds like a '60s flashback, but let's answer these age-old questions so you can find yourself.

[bob@linuxhost ~]$ whoami
bob
[bob@linuxhost ~]$ groups
bob apache

Try this, and type in your root password when prompted:

[bob@linuxhost ~]$ su -
Password:
[root@linuxhost ~]#

Several key things to notice. The prompt changed reflecting the new user (root) and the new current directory (~). The tilde means you are now in root's home directory. The dash (-) in the su command means to source the user's entire envinronment not just substitute the new user's identity. Had you left off the dash (-), the prompt would be root@www html]# instead. Minor pont at this case, but I always use su - to make sure I adopt root's PATH.

So cd /var/www/html/ to get back to where you want to work for now. Type the following commands exactly as listed:

[root@linuxhost ~]# cat > myinfo.php
<?php
phpinfo();
?>

[root@linuxhost ~]# ls -al
total 28
drwxr-xr-x 2 root root 4096 Jan 21  2009 .
drwxr-xr-x 8 root root 4096 Jul  1 13:31 ..
-rw-r--r-- 1 root root   20 Jul  1 14:22 myinfo.php

This all begs the question, so what are the (.) and (..) in the filename entries for the directories? Simple, (.) is a generic shorthand for the current directory and (..) is shorthand for one directory level up. In this case, . refers to /var/www/html/ and .. refers to /var/www/ NOTE: These are always relative paths, they always refer to the current directory and one above it. You can't move somewhere else and expect . to refer to /var/www/html/

So what do we do with this lengthy discussion? Let's change the permissions to allow user 'bob' to write into the directory.

There are several ways to enable bob, we could open up the permission and let anyone write into the directory, obviously less than optimal from a security standpoint. We could add bob to the root group, again, not a very secure thing to so. We could give bob permission to write to one single file, that's not too bad, so here's the way this will work. Let's move the new file to bob.php and give bob complete ownership of it.

[root@linuxhost ~]# cp myinfo.php bob.php
[root@linuxhost ~]# chown bob bob.php
[root@linuxhost ~]# ls -al bob.php
-rw-r--r-- 1 bob root 20 Jul  1 14:25 bob.php

So in closing, go back to the gedit editor and close the file without saving it. Open up the new file (bob.php) and modify it to your heart's content. You should be able to save it as /var/www/html/bob.php