mt_rand() gives me always the same number

I have a problem with this function always my mt_rand() give me the same number:

$hex = 'f12a218a7dd76fb5924f5deb1ef75a889eba4724e55e6568cf30be634706bd4c'; // i edit this string for each request
$hex = hexdec($hex);    
mt_srand($hex);
$hex = sprintf("%04d", mt_rand('0','9999'));

$hex is always changed, but the result is always the same 4488.

http://ift.tt/1JRnzAu