PHP中is_file不能替代file_exists的理由

复制代码 代码如下:

<?php 
      $filename = 'test.txt';
      if (is_file($filename)) {
          echo "$filename exists!/n";
      } else {
          echo "$filename no exists!/n";
      }
      sleep(10);
     if (is_file($filename)) {
          echo "$filename exists!/n";
      } else {
          echo "$filename no exists!/n";
      }
?>