PHP convert signed 64-bit integer to hex and back

Question

On a 64-bit system, off course. I would rather not use bcmath, but I think it to be necessary:

Test code, that works, but returns the output as 64-bit unsigned integer and overflows (PHP doesn't support 64-bit unsigned integers).

var_dump(-3172679146711529273);
var_dump(dechex(-3172679146711529273));
var_dump(hexdec(dechex(-3172679146711529273)));

will return:

int(-3172679146711529273)
string(16) "d3f8612029f848c7"
float(1.5274064926998E+19)

An alternative would be to pack the integer into another format (base 36?).