Try to use urlencode to encode your directory name:
$str = 'c:\\'.urlencode('表').'\t.txt'; $con=file_get_contents( $str); echo $con;
This is described here in detail.
EDIT
Assuming you're using UTF-8 encoded source files, you could also try one of the following
$str = 'c:\\'.urlencode(mb_convert_encoding('表', 'UTF-16', 'UTF-8')).'\t.txt'; // or just $str = 'c:\\'.mb_convert_encoding('表', 'UTF-16', 'UTF-8').'\t.txt';
As far as I know newer (> FAT32) Microsoft filesystems use UTF-16 encoding. But this will make your solution fail on other (e.g. Linux) filesystems.
EDIT 2
You can also try to convert your UTF-8 filename into a different encoding such as SJIS, SJIS-win, SJIS-2004, JIS, EUC-JP, eucJP-win, EUC-JP-2004, CP932, JIS-ms or the like. But I'm not an expert in east asian character encodings - so treat that information with caution.
var_dump(is_file("c:\\\\表\\t.txt"), is_readable("c:\\\\表\\t.txt"));to debug.