Will this only allow certain extensions?

I found this snippet that says will only allow certain file types. Will it work and could someone bypass it to upl开发者_如何学JAVAoad what ever file type they want? And could someone explain the substr part, i don't get how it works..

<?php
function CheckExt($filename, $ext) {
    $name = strtolower($filename);
    if(substr($name, strlen($name) -3, 3) == $ext)
        return true;
    else
        return false;
}
?>

A better way to check extensions

function checkExt($filename, $ext)
{
  $fnExt = strtolower(pathinfo($filename, PATHINFO_EXTENSION));
  if(!is_array($ext)) {
    $ext = (array)$ext;
  }
  $ext = array_map('strtolower', $ext);
  return in_array($fnExt, $ext);
}

You can then call it like

var_dump(checkExt('test.temp', 'tmp')); // false
var_dump(checkExt('test.temp', array('tmp', 'temp'))); // true

Avoid using substr as the extension length is unknown (you can use substr & strrpos as well but php provides this functionality for you)

It's very easy to bypass as changing the extension of a file does not change the contents of the file. So a .exe renamed into a .jpg is still an .exe waiting to be run anyway. You can use it for a basic check, but don't rely solely on it to validate file types.

This substr() call:

substr($name, strlen($name) -3, 3)

Is better more simply written as:

substr($name, -3)

Which PHP just interprets as 'take only the last 3 characters of $name'.

EDIT: it's not better per se because file extensions don't necessarily have to be 3 characters long. They could be 2, they could be 4, 5, even 10. This is why as I said, checking file extensions isn't very reliable.

I prefer to whitelist the Mimetypes I want to allow using something along the lines of

 $mimesGeneral = array(
        'txt'=>'text/plain',
        'doc'=>'application/msword',
        'pdf'=>'application/pdf',
        'xls'=>'application/x-excel',
        'xls'=>'application/excel',
        'xls'=>'application/vnd.ms-excel',
        'rtf'=>'application/rtf',
        'zip'=>'application/zip'

        );
$success = false;
foreach($allowedMimes as $key=>$value){

            if($_FILES['uploaded_file']['type'] == $value){

                return true;
            }
        }

I use this with a blacklist of suffixes e.g 'php', 'pl', 'exe' etc...

People will still be able to upload whatever they want; they just have to give the file a particular extension.

For substr, see the manual.