Although PHP refers to periods of time as "intervals", ISO 8601 refers to them as "durations". In ISO 8601, "intervals" are something else.
While ISO 8601 allows fractions for all parts of a duration (e.g., "P0.5Y"), DateInterval does not. Use caution when calculating durations. If the duration has a fractional part, it may be lost when storing it in a DateInterval object.