sp_columns is a MSSQL supplied stored procedure that returns information about the columns in a table. I'm still using MSSQL 6.5, but after I have established my connection I can execute the stored procedure as follows:
$query = "exec sp_columns $my_table_name"
$rs = odbc_exec($conn, $query);
It is then just a matter of stepping through the record set ($rs) to get the results.