There is the ifx_nullformat to tell wether NULL are returned as empty
strings or a string that has the text "NULL" in it.  Either forms can be
valid result values for character string result columns. There needs to
be a better solution.

Suggestion:  make Informix support work more like Oracle support that
optionally does not set the entries of the result row array returned by
ifx_fetch_row.

This could be implemented with a new function for instance named
ifx_returnnullcolumns that would take an argument that would set a an
boolean option.  null columns would only be returned if this option is true
and that would be the default value to remain compatible with the current
PHP-Informix API.

Adding this new function would be a better solution than adding an extra
mode option to ifx_nullformat because it would be easier detect if this
option is available using the function_defined() function.