PHP - 为什么system（）方法也添加标题？

Question

I was creating a php image upload script. The script sends a post request and (should) have received a json response.

This is the line for checking mime type.

$mimetype = system("file -bi -- ".escapeshellarg($_FILES["file"]["tmp_name"])); //line 63

and after few line I set the JSON header:

header('Content-Type: application/json'); //line 124

but the js does not works. On checking network tab in chrome:

Answer 1

This is because system() function writes all from the shell to script output (something like automatic echo). You can not use header() after some output was sended from script and it happened - system() sended output ("image/png; charset=binary" in network tab before warning).

Try to use exec() instead.