on a page, i have :
if (!empty($_FILES[\'logo\'][\'name\'])) {
$dossier = \'upload/\';
$fichier = basename($_FILES[\'logo\'][\'name\']);
There is a section in php documentation about file handling. You will find that you can check various errors and one of them is
UPLOAD_ERR_OK
Value: 0; There is no error, the file uploaded with success.
<...>
UPLOAD_ERR_NO_FILE
Value: 4; No file was uploaded.
In your case you need code like
if ($_FILES['logo']['error'] == UPLOAD_ERR_OK) { ... }
or
if ($_FILES['logo']['error'] != UPLOAD_ERR_NO_FILE) { ... }
You should consider checking (and probably providing appropriate response for a user) for other various errors as well.