Why does the '<<' operator sometimes behave differently in Javascript and PHP?

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

they're different because PHP is automatically converting to a long, as Python does. javascript instead gives you the low 32 bits. — jcomeau_ictx
– jcomeau_ictx, Commented Mar 24, 2011 at 4:59
splendid! So if I just & the value with 0xFFFFFFFF I get the desired value in PHP. Could you please submit that as an answer so I can accept? — Mala
– Mala, Commented Mar 24, 2011 at 5:00

jcomeau_ictx · Accepted Answer · 2011-03-24 05:03:39Z

3

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

answered Mar 24, 2011 at 5:03

jcomeau_ictx

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Comments

Andrew Cooper · Accepted Answer · 2011-03-24 05:06:00Z

1

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)

answered Mar 24, 2011 at 5:06

Andrew Cooper

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