Fast server-side rejection of large image uploads using $_FILES

Recently on his blog Cormac has posted a quick little tutorial on making things a bit faster when rejecting file uploads in PHP that are just a bit too large.

Discovered today you can report to a user if the file(s) he/she is uploading is too large without having to wait for the file to finish uploading by checking $_FILES - the $_FILES array for each form input of type “file” has an element called “error” which returns an error code without actually uploading the file if the file is larger than than upload_max_filesize in php.ini or $_POST["MAX_FILE_SIZE"]. It can do this because a “Content-length” http header is sent to the server first, and the file itself is then sent in the body of the http request.

Here’s a very simple example.

The form:

form enctype="multipart/form-data"
input type="hidden" name="MAX_FILE_SIZE" value="1048576"
input type="file" name="image"
input type="submit"
/form

The php:

if ($_FILES["image"]["error"] == UPLOAD_ERR_FORM_SIZE)
{
echo "file too big!";
}

Note that you shouldn’t just use MAX_FILE_SIZE as I’ve done above, you also need to set upload_max_filesize appropriately in php.ini