PHP pack（）返回警告和错误的十六进制

Question

我有一个密钥，我需要使用pack（）转换为HEX。

$key = "78787878787878787878787878787878787878787878787878787878787878787878787878787878787878787878787878787878787878787878787878787878";
$keyBytes = pack('H*',$key);

但是pack（）会返回：

p�������

我需要这个：

[0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38, 0x37, 0x38]

我做错了什么？我完全按照我所掌握的手册进行操作。