PHP：XOR到2个文本字符串：无法看到结果

I try to write simple xor encryption in PHP 5. But I can't see the result of xor operation. For example,

$s1="abcd";
$s2="efgh";
$res1 = $s1 ^ $s2;
$res2 = $res1 ^ $s2;
echo $res1, $res2;

displays $res2, but $res1 isn't showed. I tried to do this encryption separately for each char in string, but I got same result. As I understood, in real situation, the key and the result must to be HEX-strings.

But I can't see $res1 as HEX-number using printf. Maybe, I need to pack("H*",...) s1 and s2 strings before using XOR, but then I can't correctly unpack results (as text string and hex-number respectively).