The BSON timestamp type consists of two unsigned 32-bit values (timestamp and increment). Since PHP has no unsigned integer type, the full range of values cannot be expressed on a 32-bit platform.

Technically, we could accept signed 32-bit integers in PHP and cast them to unsigned, but this would lead to some confusing documentation and create a case where negative numbers are acceptable on a 32-bit platform but not 64-bit platforms. Instead, we should simply document the limitations on 32-bit platforms.

Note: the __toString() handler will still print the components as unsigned 32-bit integers on both platforms (per the PR for PHPC-594).