file_exists检测mount文件返回false如何解决

<?php
$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";