Trimming multi-byte spaces


would anybody know what happens behind this code?

$word = " 健一 "; // japanese character that contains two multi-byte spaces at both ends of the word

echo trim( $word, " " ); // trimming multi-byte spaces, output is expected to be "健一"

// actual output is only "健"

Is this a PHP trim bug or what? if so, why does PHP trim function include character “一” to be trimmed as well?