need to create a forget password functionality in php, mySQL

Below is where a user is able to logging in but i want to give the option to a user if they have forgotten their password by matching their username and email.

login.php

<?php



        if (!loggedin()) {


    if($_POST['submitID'] == 1){

        /* Connect to database */
        if($connectDatabase == TRUE){$action=TRUE;include('connect.php');}

        /* sanitise and check the info */
        $userName = mysql_real_escape_string($_POST['userName'],$db);
        $password = mysql_real_escape_string($_POST['password'],$db);

        if($userName == NULL) { $message = 'Please enter username.';}
        if($message == NULL && $password == NULL){ $message = 'Please enter password.';}

        if($message == NULL)
        {               
            $HashPass = md5($password);
            $userQuery = mysql_fetch_row(mysql_query("SELECT COUNT(*) FROM " . $datauser .
            " WHERE `" . $userNameField . "`='$userName' AND `" . $userPasswordField . "`='$HashPass'"));       

            /* If usercount is more than 0 -> ok */
            if($userQuery[0] > 0){

                if($connectDatabase == TRUE){$action=FALSE;include('connect.php');}

                $_SESSION['isLoged'] = 'yes';
                $_SESSION['userName'] = $userName;




                header("Location: $loginPage");
                exit();
            } else {
                $message = 'Invalid username and/or password!';
            }
        }

        if($connectDatabase == TRUE){$action=FALSE;include('connect.php');}
    }
    ?>

    <?php
    ?><h1><? echo $_SESSION['userName'];?></h1><?
    /* Display error messages */
    if($message != NULL){?>

    <?php } ?>

how will i be able to check the username from the database and allow the user to create a new password and update the database