Starting my site I've got an error

Fatal error: Call to undefined function: mssql_pconnect() in /web/sites/221/onesign/www.onesign.f2s.com/date/adodb/adodb-mssql.inc.php on line 137
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137 $this->_connectionID = mssql_pconnect($argHostname,$argUsername,$argPassword);

My site is on free server www.f2s.com

Any comments, please.