Hello there,
I want to introduce message system in my website. I've created table fields as follows:
id(int), receiver_id, msg_id, msg_sub, msg_desc, msg_time

Below is my html form in which registered users will be displayed in dropdown list:

<form class="form-horizontal" action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">
    <legend>Send Message</legend>
        Recipient:<select name="receiver_id" data-rel="chosen" onchange="showValues()">
            <?php
                $query = "SELECT * FROM star.users";
                $run = mysql_query($query);
                while( $row = mysql_fetch_assoc($run) ){
                    $receiver_id = $row['id'];
                    $usrname = $row['username'];
            ?>
            <option value="<?php echo $receiver_id; ?>" ><?php echo $usrname; ?></option>
            <?php } ?>
            </select> 
            Subject:<input type="text" name="msg_sub" value="" />
            Message:<textarea name="msg_desc" rows="4" value="" /></textarea>
             <button type="submit" name="send">Send</button>
             <button type="reset" >Cancel</button>
</form>

Now how to get receiver id from selected user and send sms to that user ?

You can use javascript function alert. For example
<body onload="alert('Your message')">

Okay, But How to send message to the selected user and store in database ?

Hello nadiam thanx for your reply, But I want to send message to the registered users inbox on this website only. Not to mobile phone.

Like one facebook user send message to another user.