What are the differences between ISO8601 and ISO8601_EXPANDED formats in PHP for date and time?

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In PHP, both ISO8601 and ISO8601_EXPANDED formats are used for representing date and time in a standardized form. However, there are some differences between the two:

1. ISO8601 format uses a simplified version of the basic format, whereas ISO8601_EXPANDED includes all elements of the basic format.

2. ISO8601 format uses a single "T" to separate date and time values, whereas ISO8601_EXPANDED uses separate delimiter characters for each element.

3. ISO8601 format allows for variations in the number of digits used for elements such as milliseconds or time zone offset, whereas ISO8601_EXPANDED has a fixed number of digits for each element.

4. ISO8601_EXPANDED format includes additional delimiters to make the format easier to read and parse.

Overall, ISO8601_EXPANDED format provides a more detailed and comprehensive representation of date and time values, whereas ISO8601 format is simpler and more compact.