为什么'<<'运算符在Javascript和PHP中的行为有时会有所不同？

Question

Take the following code snippet:

<script>
var x = 25;
document.write(x << 9);
</script>

<?php
$x = 25;
echo ($x << 9);
?>

This outputs: 12800 12800
OK. Normal so far... Now lets try this:

<script>
var x = 12345678;
document.write(x << 9);
</script>

<?php
$x = 12345678;
echo ($x << 9);
?>

This time the output is 2026019840 6320987136

Why are the latter two values different? And, most importantly (to me), how do I get the PHP implementation to do what the Javascript implementation does? In other words, I want my PHP code to output 2026019840 instead of 6320987136

Answer 1

use (x << 9) % 0x100000000 or its PHP equivalent. (or what you just said)

Answer 2

PHP is giving you a 64 bit result. Javascript is just giving you the lower 32bits. If you use (x << 9) & 0xFFFFFFFF in PHP you'll get the low 32 bits.

you may run into problems with the sign bit though. Try (23456789 << 9)