php如何返回一个image文件

The important points is that you must send a Content-Type header. Also, you must be careful not include any extra white space (like newlines) in your file before or after the <?php ... ?> tags.

As suggested in the comments, you can avoid the danger of extra white space at the end of your script by omitting the ?> tag!

<?php

　　$img = './test.png';

　　$fp =fopen($img, 'rb');

　　//send the right headers

　　header("Content-Type: image/png");

　　header("Content-Length: ".filesize($img));

　　//dump the picture and stop the script

　　fpassthru($fp);

　　exit;

?>