php - HTML Upload MAX_FILE_SIZE does not appear to work

Question

I am wondering how is the hidden field named MAX_FILE_SIZE supposed to work?

<form action="" method="post" enctype="multipart/form-data">
    <!-- in byes must preceed file field -->
    <input type="hidden" name="MAX_FILE_SIZE" value="2097152" /> 
    <input type="file" name="upload" />

    <input type="submit" name="submit" value="Submit" />
</form>

I uploaded a 4MB+ file but I got no warning from client side (I am not talking about server side). What is it MAX_FILE_SIZE supposed to do?

UPDATE

OK so its for PHP to impose a "soft" limit. But is there any difference between using it and checking something like $_FILES['upload']['size'] < 2000 in code?

Answer 1

MAX_FILE_SIZE is in KB not bytes. You were right, it is in bytes. So, for a limit of 4MB convert 4MB in bytes {1024 * (1024 * 4)} try:

<input type="hidden" name="MAX_FILE_SIZE" value="4194304" /> 

Update 1

As explained by others, you will never get a warning for this. It's there just to impose a soft limit on server side.

Update 2

To answer your sub-question. Yes, there is a difference, you NEVER trust the user input. If you want to always impose a limit, you always must check its size. Don't trust what MAX_FILE_SIZE does, because it can be changed by a user. So, yes, you should check to make sure it's always up to or above the size you want it to be.

The difference is that if you have imposed a MAX_FILE_SIZE of 2MB and the user tries to upload a 4MB file, once they reach roughly the first 2MB of upload, the transfer will terminate and the PHP will stop accepting more data for that file. It will report the error on the files array.