How to exclude file only from root folder in Git

I am aware of using .gitignore file to exclude some files being added, but I have several config.php files in source tree and I need to exclude only one, located in the root while other keep under revision control.

What I should write into .gitignore to make this happen?

From the documentation:

If the pattern does not contain a slash /, git treats it as a shell glob pattern and checks for a match against the pathname relative to the location of the .gitignore file (relative to the toplevel of the work tree if not from a .gitignore file).

A leading slash matches the beginning of the pathname. For example, "/*.c" matches "cat-file.c" but not "mozilla-sha1/sha1.c".

So you should add the following line to your root .gitignore:

/config.php

Use /config.php.

Older versions of git require you first define an ignore pattern and immediately (on the next line) define the exclusion. [tested on version 1.9.3 (Apple Git-50)]

/config.php
!/*/config.php

Later versions only require the following [tested on version 2.2.1]

/config.php

If the above solution does not work for you, try this:

#1.1 Do NOT ignore file pattern in  any subdirectory
!*/config.php
#1.2 ...only ignore it in the current directory
/config.php

##########################

# 2.1 Ignore file pattern everywhere
config.php
# 2.2 ...but NOT in the current directory
!/config.php