파일업로드시 확장자 체크

$valid_extensions = array('jpg', 'gif', 'png');

$file_name = basename($_FILES['userfile']['name']);
$_file_name = explode('.', $file_name);
$ext = $_file_name[ count($_file_name) - 1 ];

if( !in_array($ext, $valid_extensions) ) {
/* This file is invalid */
}