Manual:32-bit

Known issues
Known issues with 32-bit PHP:
 * large values in 'bigint' fields in the databases, such as row IDs for very large sites, will fail to be interpreted correctly.
 * This can affect folks importing Wikipedia databases to local installations.
 * uploaded files larger than 4 gigabytes are not possible
 * available memory usage may be limited
 * Dates in StructuredDiscussions are wrong, displaying "47 years ago" everywhere.

Is it safe to ignore?
As of 2017, if you're running a small or modestly-sized site and not importing any data from Wikipedia, you may not actually encounter any problems from using a 32-bit PHP and it will probably work.

However be aware that at some point this may change, and 64-bit servers may be required in future.

How to fix it

 * If your CPU is 32-bit, you're stuck with 32-bit PHP.
 * If your CPU is 64-bit but your operating system is 32-bit, you must update to a 64-bit operating system.
 * If your operating system is 64-bit but your PHP is 32-bit (this is most common on Windows), you may need an alternate PHP installer. If there is a separate 64-bit installer where you got your PHP, choose that.
 * Warning: XAMPP is known (as of October 2017) to only provide a 32-bit build for Windows. Bitnami WAMP is known to have working 64-bit builds.