Description:
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I was pulling my hair out getting the error:
  "stored procedure execution failed"
when trying to run mssql_execute but could run:
  $results = mssql_query('sp_test'); 
fine.

I was connecting using the string:
  $dbserver="xxx.xxx.xxx.xxx:1433";
  $cn = mssql_connect($dbserver, $dbuser, $dbpass);

where xxx is the IP address.

It seems that this doesn't work, you need to do the following:
edit your freetds.conf file and add the connection in here, eg:
[YourServer]
        host =xxx.xxx.xxx.xxx
        port = 1433
        tds version = 8.0

Then try and connect as:
$cn = mssql_connect('YourServer', $dbuser, $dbpass);

This could be made clearer in the docs.  I've added this as a note as well.  There are a couple of bug reported about this, which if the docs were clearer wouldn't get reported hopefully.