Edit freetds.conf (uncomment and change version):

from

[global]
        # TDS protocol version
;   tds version = 4.2

to

[global]
        # TDS protocol version
    tds version = 7.0

and restart apache.

Source: http://stackoverflow.com/questions/13180746/mssql-connect-unable-to-connect-to-server-without-freetds-conf