Self-taught php has a little question

Self-taught php has a small problem to ask experts & nbsp; test. php & lt ;? Phpecho & nbsp; filesize (localhost: 81qq.jpg); echo & nbsp; filesize (E: Apachehtdocsqq.jpg );? & Gt; Why is this filesize function so fond of reporting warning? Warni self-learning php has a small problem.
Test. php
// Echo filesize ("http: // localhost: 81/qq.jpg /");
Echo filesize ("E:/Apache/htdocs/qq.jpg /");
?>
Why is this filesize function so fond of reporting warning,
Warning: filesize () [function. filesize]: stat failed for E:/Apache/htdocs/qq.jpg/in E: \ Apache \ htdocs \ test. php on line 4

The apache path is correct.
------ Solution ----------------------
Filesize ("E:/Apache/htdocs/qq.jpg /");
What does the last '/' mean?