PHP bitwise left shifting 32 spaces problem and ba

问题:

I have the following problems:

First: I am trying to do a 32-spaces bitwise left shift on a large number, and for some reason the number is always returned as-is. For example:

echo(516103988<<32); // echoes 516103988

Because shifting the bits to the left one space is the equivalent of multiplying by 2, i tried multiplying the number by 2^32, and it works, it returns 2216649749795176448.

Second: I have to add 9379 to the number from the above point:

printf('%0.0f', 2216649749795176448 + 9379); // prints 2216649749795185920 

Should print: 2216649749795185827

回答1:

Php integer precision is limited to machine word size (32, 64). To work with arbitrary precision integers you have to store them as strings and use bc or gmp library:

   echo bcmul('516103988', bcpow(2, 32));  // 2216649749795176448

回答2:

Based on Pascal MARTIN's suggestions, i tried both the BCMath and the GMP extension and came up with the following solutions:

With BCMath:

$a = 516103988;
$s = bcpow(2, 32);    
$a = bcadd(bcmul($a, $s), 9379);
echo $a; // works, echoes 2216649749795185827

With GMP:

$a = gmp_init(516103988); 
$s = gmp_pow(gmp_init(2), 32); 
$a = gmp_add(gmp_mul($a, $s), gmp_init(9379)); 
echo gmp_strval($a);  // also works

From what i understand, there is a far greater chance for BCMath to be installed on the server then GMP, so i will be using the first solution.

Thanks :)

回答3:

Doing 32 bit-shifting operations will probably not work like you expect, as integers tend to be stored on 32 bits.

Quoting this page : Bitwise Operators

Don't right shift for more than 32 bits on 32 bits systems.
Don't left shift in case it results to number longer than 32 bits.
Use functions from the gmp extension for bitwise manipulation on numbers beyond PHP_INT_MAX.