CODEWITHSUNDEEP
phpintegerbit-manipulationbit
gsirin
gsirin

Reputation: 41

Unexpected result in PHP bitwise operation

The operation 1539 | 0xfffff800 returns -509 in JavaScript and Python 2.7. In PHP I get 4294966787.

Does anybody know why and could explain that to me. I would love to know how I get the expected result in PHP as well.

Upvotes: 4

Views: 147

Answers (1)

TomS
TomS

Reputation: 511

1539 | 0xfffff800 = 4294966787 (= 0xFFFFFE03) This is perfectly right. So, PHP is right.

If you would like to have both positive and negative integers, you need some mechanism to determine whether the number is negative. This is usually done using the 2-complement of the number. You can negate a number by just inverting all the bits of the number and then add 1 to it. In order to avoid ambiguities, you cannot use all the bits of your integer variable. You cannot use the highest bit in this case. The highest bit is reserved as a sign bit. (If you would not do so, you never know if your number is a big positive number or a negative number.)

For exammple with an 8 bit integer variable, you would be able to represent numbers from 0 to 255. If you need signed values, you can represent number from -128 (1000 000 binary) to +127 (0111 1111).

In your example, you have a 32 bit number which has its highest bit set. In Python and JavaScript, it's interpreted as negative number, as they apparently have 32 bit variables, and there, the highest bit is set. They interpret that as negative number. So, the result of your calculation is also negative. In the PHP version you are using, the integer variable seems to be 64 bit long and only the lower 32 bits are used. The highest bit (bit 63) is not set, so PHP interprets this number as positive. Depending on what you want to achive, you may want to fill up all bits from bit 32 to bit 63 with 1s which will create a negative number...

Upvotes: 1

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