To use the MSSQL extension on Unix/Linux, you first need to build and install the FreeTDS library. FreeTDS is a set of libraries for Unix and Linux that allows your programs to natively talk to Microsoft SQL Server and Sybase databases.

You can refer to the following url to get informations about mssql extension and  the functions supported with it.
‘http://www.php.net/manual/en/book.mssql.php‘

To get these functions to work, you have to compile PHP with –with-mssql[=DIR] , where DIR is the FreeTDS install prefix. And FreeTDS should be compiled using
–enable-msdblib

Installation instruction:-

(This is not specific to cpanel servers, you can recompile php directly without easyapache in any normal linux servers with ‘–with-mssql=/usr/local/freetds’)

1-. Download freetds source -> www.freetds.org
2-. tar -zxvf freetds-stable-tgz
3-. cd freetds-*
4-. ./configure –prefix=/usr/local/freetds –with-tdsver=8.0 –enable-msdblib  –with-gnu-ld

Note: If you use SQL 2000, 2005, 2008 then tds version = 8.0
if you use SQL 7.0 then tds version = 7.0

5-. make
6-. make install
7-. To check freetds working, run from terminal
7.1 /usr/local/freetds/bin/tsql -S <ip of the server> -U <User SQL>
7.2 If the connection parameters are correct you will be connected to a prompt.
8-. Add the following text in freetds.conf ( /usr/local/freetds/etc )
[TDS]
host = <ip of the Server with Sql>
port = 1433
tds version = 8.0

9-. Add the following line in the file /etc/ld.so.conf and run ldconfig -v:
include /usr/local/freetds/lib

10-. Recompile PHP with –with-mssql=/usr/local/freetds flag.
Create a file called all_php4 or all_php5 in:

‘/var/cpanel/easy/apache/rawopts/’

The file doesnt exist by default, just create it and add this line to the file:
–with-mssql=/usr/local/freetds
Easy apache will check this file ‘/var/cpanel/easy/apache/rawopts/all_php5’ for any additional php configuration option during recompilation.

11-. Run ‘/scripts/easyapache’

Notes:-

1-. If you are running a 64bit OS and get an error about configure: error: ‘Could not find /usr/local/freetds/lib64/libsybdb.a|so’
then you need to do the following:
1.1 Make sure libsybdb.so is available on the server,then
1.2 ln -s /usr/local/freetds/lib/libsybdb.so.5 /usr/lib64/libsybdb.so.5
1.3 ln -s /usr/local/freetds/lib/libsybdb.so.5 /usr/local/freetds/lib64/libsybdb.so
1.4 Run   ldconfig -v

2-. If you are getting an error about configure:’ error: Directory /usr/local/freetds is not a FreeTDS installation directory’
2.1 cp [tds source]/include/tds.h /usr/local/freetds/include
2.2 cp [tds source]src/tds/.libs/libtds.a /usr/local/freetds/lib
2.3 Recompile again.

Reference:-

http://www.freetds.org/userguide/
http://www.php.net/manual/en/book.mssql.php
http://forums.cpanel.net