$user_check = mysql_query("SELECT userdb_user_name FROM default_userdb WHERE userdb_user_name='$reciever'");
  $user_check = mysql_num_rows($user_check);
 
Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /xx/xx/x/xx/xx/xx/xxxx/xxxxx/htdocs/xxxx/xxx/xxx.php

if (!$user_check) {
    die('Invalid query: ' . mysql_error());
}

if (!$user_check) {
    die('Invalid query: ' . mysql_error());
}

<?php
session_start();
$user = $_SESSION['username'];
$reciever = $_REQUEST['reciever'];
 
    include 'db.php';
 
    //This checks to see if a user is logged in or not by seeing if the sessioned username varialble exists.
	//You could change this check to however you want to validate your members, this is just how I did it.
    if(!$user)
        {
        echo "<br><p>Blah blah you arent logged in and stuff, you should do that or something</p><br>";
        }
 
    else
        {
        //Query the database to see how many messages the logged in user has, then do a little math
		//Find the percentage that your inbox is full (message count divided by 50)
		//50 messages maximum, you can change that
		$sql = mysql_query ("SELECT pm_count FROM default_userdb WHERE userdb_user_name='$user'");
		$row = mysql_fetch_array ($sql);
		$pm_count = $row['pm_count'];
 
		//This is the math to figure out the percentage.
		//The message could divided by 50 then multiplied by 100 so we dont have a number less than 1
		$percent = $pm_count/'50';
		$percent = $percent * '100';
        ?>
        <br>
        <center>
        <b><p><a href="inbox.php">Inbox</a> | <a href="compose.php">Compose</a> | <a href="sent.php">Sentbox</a></b>
        <b><p><?php echo "$pm_count"." of 50 Total  |  "."$percent"."% full"; ?></p></b>
        </center>
        <br>
        <?php
        //So here we get the variable submitted through the form to this page
        if($reciever == NULL)
        {
        $reciever = $_POST['username'];
        }
        $subject = $_POST['subject'];
        $message = $_POST['message'];
        $error = '0';
 
        //If they are all blank we jsut say to compose a message
        if(!$reciever AND !$subject AND !$message)
            {
            ?>
            <p><b>Please compose a message.</b></p>
            <br>
            <?php
            }
 
        //Since this form was partially filled out we need to return an error message
        else
            {
            if (!$reciever)
                {
                $error = 'You must enter a reciever to your message';
                }
 
            if (!$subject)
                {
                $error = 'You must enter a subject';
                }
 
            if (!$message)
                {
                $error = 'You must enter a message';
                }
 
            //If the variable error is not set to zero, we have a problem and should show the error message
            if($error != '0')
                {
                echo "<p>$error</p><br>";
                }
 
            //There are no errors so far which means the form is completely filled out    
            else
                {
                //Are the trying to send a message to a real user or to something they just made up?
                $user_check = mysql_query("SELECT userdb_user_name FROM default_userdb WHERE userdb_user_name='$reciever'");
                $user_check = mysql_num_rows($user_check);
 
                //The user is real and not made up if this is true
                if($user_check > '0')
                    {
                    //There might already be a sessioned time variable, if so we need to get it for the flood check
                    $time = $_SESSION['time'];
 
                    //If there is a time variable already, set it to the varialbe $old_time
                    if($time > '0')
                        {
                        $old_time = $time;
                        }
 
                    //Here we get the minutes and seconds on the server time using the date function, and set that to the $time variable
                    //Now we find the difference between this time ($time) and the time that the page was submitted ($old_time)
                    $time = date('is');
                    $difference = $time - $old_time;
 
                    $_SESSION['time'] = $time;
 
                    //If the two times have a difference greater or equal to 15, which is 15 seconds, they can submit the message, this is for flood protection
                    if($difference >= '15')
                        {
                        //Get their private message count
                        $sql = mysql_query ("SELECT pm_count FROM default_userdb WHERE username='$reciever'");
                        $row = mysql_fetch_array ($sql);
                        $pm_count = $row['pm_count'];
 
                        //You cant have more than 50 private messages, if they try sending a message to a user with a full inbox return an error message
                        if(pm_count == '50')
                            {
                            $error = 'The user you are trying to send a message to has 50 private messages, sorry but we cant send your message untill that user deletes some of their messages.';
                            }
 
                        else
                            {    
                            //And now we stick the message in the database with all the correct information
                            mysql_query("INSERT INTO messages (reciever, sender, subject, message) VALUES('$reciever', '$user', '$subject', '$message')") or die (mysql_error());
							//Add 1 to the pm count, update the reciever with the new pm count
							$pm_count++;
							mysql_query("UPDATE default_userdb SET pm_count='$pm_count' WHERE username='$reciever'");
                            }
 
                        //Let the user know everything went ok.
                        echo "<p><b>You have successfully sent a private message!</b></p><br>";
                        }
 
                    //Since they are trying to send messages faster than every 15 seconds, give them an error message    
                    else
                        {
                        $error = 'You must wait 15 seconds before sending another private message';
                        }
                    }
 
                //If they mis spelled or, made up a username, then give an error message telling them its wrong.
                else
                    {
                    $error = 'That username does not exist, please try again. Remember to check your spelling, and don\'t make stuff up at random.';
                    }
                }
            }
 
        //Since we may have set the error variable to something while trying to send the messae, we need another error check
        if($error != '0')
            {
            echo "<p>$error</p><br>";
            }
 
        //else
          //  {
            //Here's the form for the input
            ?>
            <form name="send" method="post" action="compose.php">
            <table width="80%">
              <tr>
                <td width="150px" align="left" valign="top"><p>Username</p></td>
                <td width="" align="left" valign="top"><input name="username" type="text" id="username" value="<?php echo "$reciever"; ?>"></td>
              </tr>
 
              <tr>
                <td width="150px" align="left" valign="top"><p>Subject</p></td>
                <td width="" align="left" valign="top"><input name="subject" type="text" id="subject" value="<?php echo "$subject"; ?>"></td>
              </tr>
 
              <tr>
                <td width="150px" align="left" valign="top"><p>Message Body</p></td>
                <td width="" align="left" valign="top"><textarea name="message" type="text" id="message" value="" cols="50" rows="10"></textarea></td>
              </tr>
 
              <tr>  
                <td></td>
                <td><input type="submit" name="Submit" value="Send Message"></td>
              </tr>
            </table>
            </center>
            </form>
            <?php
            }
        //}    
    ?>

if (!$user_check) {
die('Invalid query: ' . mysql_error());
}

$user_check = mysql_query("SELECT userdb_user_name FROM default_userdb WHERE userdb_user_name='$reciever'");

                $user_check = mysql_num_rows($user_check);

Invalid query: Table 'newokul_1.default_userdb' doesn't exist