Unicode不是真正的编码；它是总体标准的名称，主要由Microsoft用作UTF-16BE的别名，显然，PHP为此支持它。您期望的是UTF-16LE，因此请明确使用它：

"Unicode" is not a real encoding; it's the name of the overarching standard and used as an alias for UTF-16BE mostly by Microsoft, and apparently PHP supports it for that reason. What you expect is UTF-16LE, so use that explicitly:

$source = mb_convert_encoding('test', 'UTF-16LE', 'UTF-8');