Here i am trying to insert a query for register the user in my site

any body can tell me dat wts the mistake is there

PHP Code:
<?php
require("settings.php");
// Register information 
if ($_POST['submit']!="") {
    if($_POST['userid']!=""){
        if ($_POST['username'] != "") {
            if ($_POST['password'] != "") {
                $userid = mysql_real_escap_string($_POST['userid']);
                $username = mysql_real_escape_string($_POST['username']);
                $password = mysql_real_escape_string($_POST['password']);
                echo $userid;
                echo $username;
                echo $password;
                
                $password = md5($password);//To stroe the password in md5
                $sql =(" INSERT INTO 'users' (userID, username, password) VALUES('$userid','$username,'$password') ");
                $query = mysql_query($sql);
                echo "Register Successful";
                } 
                else {
                    echo "Register Failed";
                }
                }
                    else {
                        echo "Please enter username";
                    }
                
                }
                    else {
                        echo "please enter the password";
                    }
                }
                
                
?>
<html>
<body>
<font color="#FF0000"></font>  
<form action="register.php" method="post"> <br>
Userid:<input type="text" name="userid"><br>
Username:<input type="text" name="user"><br>
Password:<input type="password" name="pass"><br>
<input type="submit" name="Register" value="Register" ><br>
</body>
<html>
Please help me