无法在服务器上传文件 - PHP

I'm trying to upload a file from a multi-upload form. Despite the fact that the $target_file output displays the correct filepath the file doesn't being uploaded to server. Here is the code:

<?php
for ($i = 0; $i < count($_FILES['file']['tmp_name']); $i++) {
        $target_dir = '../conference/files/'. $row['CONFERENCE_ID'] .'/';
        $target_file = $target_dir . basename($_FILES['file']['name'][$i]);
        $uploadOk = 1;
        $fileTypeExtension = pathinfo($target_file,PATHINFO_EXTENSION);
        // Check if file already exists
        if (file_exists($target_file)) {
            echo "Sorry, file already exists.";
            $uploadOk = 0;
        }
        // Check if $uploadOk is set to 0 by an error
        if ($uploadOk == 0) {
            echo "Sorry, your file was not uploaded.";
        // if everything is ok, try to upload file
        } else {
            if (move_uploaded_file($_FILES["file"]["name"][$i], $target_file)) {
                echo "The file ". basename( $_FILES['file']['name'][$i]). " has been uploaded.";
            } else {
                echo "Sorry, there was an error uploading your file. ";
                echo $target_file;
            }
        }
}
?>

UPDATE: I add this code to check if files are uploaded and the result is no. Any ideas why that happens?

if (is_uploaded_file($_FILES['file']['name'][$i])) {
            echo "Temp file uploaded. 
";
} else {
            echo "Temp file not uploaded. 
";
}

UPDATE 2: I tried to upload only one file and it works fine. The problem is with multiple files. Can xampp version be the problem?