If you use an argument, make sure its the CONNECTION_ID and not the RESULT_ID.
Testing the result can return a null string or sometimes a garbage string.
# -- Example code --
$rs = odbc_exec($dbc, $sql);
#this is wrong but won't error out until
#you demo the page for a client!
if (odbc_error($rs)) die(...);
#these work
if (odbc_error()) die(...);
if (odbc_error($dbc)) die(...);