Weird Undefined offset: 0 error - php

Question

I've the following PHP code that receives a file from an iOS app.
The information is received and for my debuggin $file is setted and has content in all its forms. The original value passed is btn-7.png and $file[0] = btn-7.
The function is processed and all images are copied to the system as I see them in the folder.... thus I dont know why i am getting this error.
The error appear in the first line available to the function that shows $_FILES[$file]
Does anyone have an idea why this is happening?
EDIT: As I said. No need to do var_dump because I can echo $file. Also I am debbugin from iOS console can't see non-JSON results, I only rely on PHP logs to know the actual error message.
EDIT2: as requested `var_dump(); (I am working from XCode, iOS dev console, not web browser)
ob_start();
var_dump($file);
$e = ob_get_contents();
ob_end_clean();
error_log($e, 0);
the result
[06-Jun-2013 17:58:26 UTC] string(9) "btn-7.png"`
the error
[06-Jun-2013 17:24:45 UTC] PHP Notice: Undefined offset: 0 in C:\xampp\htdocs\igym\classes\shareexercise.php on line 146
PHP
private function storeFile( $file )
{
$file = explode('.',$file);
$file = $file[0];
$msg['asd'] = $file;
echo json_encode($msg);
if( $_FILES[$file]["type"] !== "image/png" && $_FILES[$file]["size"] < 2048600 )
{
$error['error'] = 'There was a problem uploading your picture';
echo json_encode($error);
return 0;
}
if( move_uploaded_file( $_FILES[$file]['tmp_name'], IMGUPLOADDIR . '/' . $this->userID . '/' . $_FILES[$file]['name'] ) ) {
return true;
} else{
//$error['error'] = 'There was a problem uploading your picture';
//echo json_encode($error);
return false;
}
}

Answer 1

$_FILES is an associative array, not a numerical array:
From: php.net
$_FILES
An associative array of items uploaded to the current script via the HTTP POST method.
You should var_dump($_FILES) and see what the structure looks like. 0 doesn't exist because the index is the field name or some other string.
Aso: A tutorial on $_FILES