file_exists检测mount文件老是返回false

$source= '/mnt/images/554-100006.jpg';
$target = '/tmp/'.basename($source);

echo "SOURCE: {$source}\n";
echo "TARGET: {$target}\n";

echo "FILE_EXISTS: ".(file_exists($source)?'TRUE':'FALSE')."\n";
echo "IS_FILE: ".(is_file($source)?'TRUE':'FALSE')."\n";
echo "COPY: ".(copy($source, $target)?'TRUE':'FALSE')."\n";

执行功效：

SOURCE: /mnt/images/554-100006.jpg
TARGET: /tmp/554-100006.jpg
FILE_EXISTS: FALSE
IS_FILE: FALSE
COPY: TRUE