Question

I’m having a problem with converting binary strings to signed integers

If you call decbin(‘-40’), php will output 1111111111111111111111111111111111111111111111111111111111011000

But if you call bindec(decbin(‘-40’)), it will output 1.84467440737E+19 (or something similar, which is obviously not -40) because it “sees the most significant bit as another order of magnitude rather than as the sign bit” – php manual

Is there a way to convert a binary 64 bit binary string (much like the one output by decbin) string into a signed integer?

Answer 1

From the documentation, you cannot use bindec

bindec() interprets all binary_string values as unsigned integers. This is because bindec() sees the most significant bit as another order of magnitude rather than as the sign bit.

base_convert appears to ignore signing altogether.

If you know that your incoming string will always be 64 bit binary and you are not on a 32 bit system, it’s quite easy to write a custom function.

• Check if the string is 64 characters long.
• Check if the most significant bit is a 1.
• Flip all bits
• Add 1
• Negate

Here’s a quick one I knocked together.

function bindec2($bin)
{
    if (strlen($bin) == 64 && $bin[0] == '1') {
        for ($i = 0; $i < 64; $i++) {
            $bin[$i] = $bin[$i] == '1' ? '0' : '1';
        }

        return (bindec($bin) + 1) * -1;
    }
    return bindec($bin);
}