This RFC improves cross-platform consistency in PHP for some operations dealing with integers, and makes PHP's behaviour more intuitive, as well as partly paving the way for Big Integer Support .

Right bitwise shifts by a number of bits beyond the bit width of an integer will always result in 0 or -1 (depending on sign), even on CPUs which wrap around

Left bitwise shifts by a number of bits beyond the bit width of an integer will always result in 0, even on CPUs which wrap around

Instead of being undefined and platform-dependant, NaN and Infinity will always be zero when casted to integer

For all “was” results, these are only on my machine, as they would differ across platforms. All “now” results are the same on all platforms.

This RFC tries to clean up some integer edge cases and make integers more consistent across platforms. As PHP is a high-level language, we ought to abstract away implementation differences, otherwise we make it difficult to write code that runs consistently across platforms, something which is an key requirement for PHP.

“Negative shifts” do not do what users would reasonably expect them to do: shift in the opposite direction. Rather, a negative shift is usually a shift by the 2's complement unsigned integer representation (in the case of -2 , this would be 18446744073709551614 when integers are 64-bit). This is also reliant on undefined behaviour in C, and will give different results depending on the processor and integer size. For this reason, we now disallow such shifts.

On Intel CPUs, a bitwise shift by a number of bits that is greater than the bit width of an integer (e.g. >> 65 on a 64-bit machine) will “wrap around” (e.g. >> 65 is effectively >> 1 ). To ensure cross-platform consistency, we ensure that such shifts will always result in zero (for left shifts), or zero or negative one (for right shifts, depending on the sign of the number being shifted). It is worth noting that shifts of a number of bits greater than the bit width of an integer is also undefined behaviour in C.

Making NaN and Infinity always become zero when casted to integer means more cross-platform consistency, and is also less surprising than what is currently produces, where NaN produces the minimum integer on my machine ( -9223372036854775808 ).