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<?php
$formaction = $_SERVER['PHP_SELF']; //the URL the form should send users to on submit
$allowedfiletypes = array('csv'); //a comma separated list of allowed extensions
$uploadfolder = './'; //the folder where the uploaded file should be moved to
$uploadfilename = 'data.csv'; //the filename that the uploaded file should be renamed to

//check to see if the form has been submitted
if (array_key_exists('action',$_POST) && is_string($_POST['action']) && (strip_tags($_POST['action']) == 'upload')) { //the form has been submitted
    echo '<p>Uploading file... ';
    if (empty($_FILES['uploadfile']['name'])) {
        echo '<strong>Error: File not uploaded!</strong></p>';
    } else {
        $fileext = strtolower(substr($_FILES['uploadfile']['name'],strrpos($_FILES['uploadfile']['name'],'.')+1));
        if (!in_array($fileext,$allowedfiletypes)) {
        	echo '<strong>Error: Invalid file extension!</strong></p>';
        } else { //a file of the correct type was uploaded
            $fulluploadfilename = $uploadfolder.$uploadfilename;
            if (move_uploaded_file($_FILES['uploadfile']['tmp_name'], $fulluploadfilename)) {
                echo 'Your file has been uploaded succesfully:<br>'.$fulluploadfilename.'</p>';
            } else {
            	echo '<strong>Error: Could not save file:<br>'.$fulluploadfilename.'</strong></p>';
            }
        }
    }
}

echo '<form action="'.$formaction.'" method="post" enctype="multipart/form-data">';
echo '<p>Your CSV file:<br><input type="file" name="uploadfile"></p>';
echo '<p><input type="submit" class="button" value="Upload File"></p>';
echo '<input type="hidden" name="action" value="upload">';
echo '</form>';