PHP bitwise operations not returning correct values

Question

I have a function which takes a numbers and returns an array corresponding to the days (the number will be bit-masked for each day of the week). However the array is returning all the days for certain values and an empty array for another values. Below is the function

function get_days($days) {
    $days_arr = array();


echo "days: " . decbin($days) . " - type: " . gettype($days) . "<br/>";
echo "type1: " . gettype($days & 0x01) . " - type2: " . gettype(0x01) . "<br/>";
echo "days & 0x01 = " . dechex($days & 0x01) . " = " . ($days & 0x01 == 0x01) . "<br/>";
echo "days & 0x02 = " . dechex($days & 0x02) . " = " . ($days & 0x02 == 0x02) . "<br/>";
echo "days & 0x04 = " . dechex($days & 0x04) . " = " . ($days & 0x04 == 0x04) . "<br/>";
echo "days & 0x08 = " . dechex($days & 0x08) . " = " . ($days & 0x08 == 0x08) . "<br/>";
echo "days & 0x10 = " . dechex($days & 0x10) . " = " . ($days & 0x10 == 0x10) . "<br/>";


    if($days & 0x01 == 0x01)
        $days_arr[] = 'M';

    if($days & 0x02 == 0x02)
        $days_arr[] = 'T';

    if($days & 0x04 == 0x04)
        $days_arr[] = 'W';

    if($days & 0x08 == 0x08)
        $days_arr[] = 'H';

    if($days & 0x10 == 0x10)
        $days_arr[] = 'F';

    return $days_arr;
}

Below are the results of the echo

days: 10101 - type: integer
type1: integer - type2: integer
days & 0x01 = 1 = 1
days & 0x02 = 0 = 1
days & 0x04 = 4 = 1
days & 0x08 = 0 = 1
days & 0x10 = 10 = 1
days: 1010 - type: integer
type1: integer - type2: integer
days & 0x01 = 0 = 0
days & 0x02 = 2 = 0
days & 0x04 = 0 = 0
days & 0x08 = 8 = 0
days & 0x10 = 0 = 0

I can't seem to figure the reason behind the issue, it seems logical to me that this should work.

This is an operator precedence issue. The bitwise expression should be parenthesized because the comparison has higher precedence so it happens before the bitwise operation. In your if statements, the syntax that gives you the desired result would be if( ($days & 0x08) == 0x08). See php.net/manual/en/language.operators.precedence.php And php.net/manual/en/language.operators.bitwise.php — drew010
– drew010, Commented Apr 16, 2017 at 17:21

KIKO Software · Accepted Answer · 2017-04-16 17:22:22Z

1

This is an operator precedence issue. See:

http://php.net/manual/en/language.operators.precedence.php

So == is above &. You should not do:

$days & 0x02 == 0x02

But:

($days & 0x02) == 0x02

answered Apr 16, 2017 at 17:22

KIKO Software

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