links
You have noticed on RedHat & CentOS, there is no PHP-MSSQL module/extension available in the default yum repositories.
So if you want to use it, you can search php-mssql and download it from the follwoing URL. You do not configure ODBC or use sybase extension.
Example :
| # yum install php-mssql-5.1.6-5.el5.x86_64.rpm |