calculating xor checksum

[rajoo.sharma]

hello everyone,

 

Could you please help me on calculating XOR checksum.

 

The following works fine and gives me 0, which means the data is correct and we know by manually calculating it that it is correct. But I'm not able to implement it using a loop, as I really have no idea how long the value will be, it can be just 4 characters long or 100.

Characters means hex values, a set of 2 each. so in a 4 characters long string we have 2 hex values i.e. a01e will be treated as a0 and 1e.

 

Following works fine and gives me 0:

 

function checksum($values){

$stra=substr($values, 0, 2);

$strb=substr($values, 2, 2);

$strc=substr($values, 4, 2);

$strd=substr($values, 6, 2);

$stre=substr($values, 8, 2);

$strf=substr($values, 10, 2);

$strg=substr($values, 12, 2);

$strh=substr($values, 14, 2);

$stri=substr($values, 16, 2);

eval ("\$a = 0x$stra;\$b = 0x$strb;\$c = 0x$strc;\$d = 0x$strd;\$e = 0x$stre;\$f = 0x$strf;\$g = 0x$strg;\$h = 0x$strh;\$i = 0x$stri;");

echo $a^$b^$c^$d^$e^$f^$g^$h^$i;

}

$values="0191050902040843d1";

checksum($values);

 

 

Following gives 2162888:

 

function checksum($values){

$stra=substr($values, 0, 2);

$i=2;

while($i<strlen($values))

{

$strb=substr($values, $i, 2);

$i+=2;

eval ("\$a = 0x$stra;\$b = 0x$strb;");

$c=$a^$b;

$stra=$c;

}

echo $stra;

}

$values="0191050902040843d1";

checksum($values);

 

Please Help

Regards