Interbase 6/Firebird generators are always 64-bit signed integers
(Firebird's BIGINT type), regardless of the platform the client is
running.
On 64-bit platforms, this will never require conversion as PHP's int
type is 64 bits wide on those platforms. On 32-bit platforms, the
generated value is converted to a string if it exceeds the limits of a
32-bit signed integer.
(This behaviour is consistent with the ibase_fetch_*() functions, that
will use strings to represent numerical values that cannot be represented
accurately using native PHP numerical types.)