Difference in the results of operations bitwise between javascript and php

Question

Javascript code:

console.log( 1 << 5 );
console.log( 1111044149 << 2 );

Javascript output:

32
149209300

PHP code:

var_dump( 1 << 5 );
var_dump( 1111044149 << 2 );

PHP output:

32
4444176596

Why 1111044149 << 2 has difference between PHP and javascript? I'm writing a javascript code to PHP and everything worked less that part and I need the PHP code shows similar results obtained in javascript to be perfect.

possible duplicate of [Bitshift in javascript](http://stackoverflow.com/questions/2373791/bitshift-in-javascript) — Gábor Bakos, Apr 19 '15 at 06:22

ProgramFOX · Answer 1 · 2015-04-19T06:33:47.093

3

The operands of a bitwise operation in JavaScript are always treated as int32, and in PHP this is not the case. So, when performing the left shift on 1111044149, this happens in JS:

01000010001110010011000000110101 (original, 32-bit)

00001000111001001100000011010100 (left shifted 2 positions, "01" is truncated)
= 149209300

And in PHP, the bits do not get truncated because it is not treated as a 32-bit integer.

edited Apr 19 '15 at 06:33

answered Apr 19 '15 at 06:20

ProgramFOX

6,131
11
45
51

Actually, JS numbers are 64b floats. If the php output had been wrong, it would have been easy to explain due to overflow. – Jite Apr 19 '15 at 06:22
1

@Jite Indeed they are, but bitwise operations in JS work with 32-bit integers. – ProgramFOX Apr 19 '15 at 06:24
@ProgramFOX I had no idea of this. Thank you for info! :) – Jite Apr 19 '15 at 06:25

score 0 · Answer 2 · answered Apr 19 '15 at 06:52

Thanks for the explanations, helped me a lot and I could now make a function to handle this automatically conversion, I had forgotten that detail about 32-bit and 64-bit for lack of attention.

Function

function shift_left_32( $a, $b ) {
    return ( $c = $a << $b ) && $c >= 4294967296 ? $c - 4294967296 : $c;
}

Test

var_dump( shift_left_32( 1, 5 ) );
var_dump( shift_left_32( 1111044149, 2 ) );

Output

32
149209300

Difference in the results of operations bitwise between javascript and php

2 Answers2

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