we have two modules in our project one is customer and another service provider.......

the customer based on services that the service provider is providing he will select and send a quote ......actually now customer is able to see the services and can send a quote.....if the customer sees and send it to the service provider.....it should like mail box to the service provider so that he can check who are all the customer who are sent a quote.....how to create an mail box.....

you just create a table like messages in your database, and create a sent form , insert customer quotes in database including customer id which is in session,,
And at service provider side,you just retrieve them to his box means:

function getdata($tble,$field,$whr) { 
 $q="SELECT $field FROM $tble where $whr";
return $r[0];
<a href="inbox.php?read" >inbox(<?=$msgcount; ?>)</a>

You have place a field like status,then whenever it is 0,make the message as unread..otherwise read..

When ever user clicks on that message:
you have make the status as 1.

By using this function:

update messages set status=1 where senderid=$_SESSION['user_id'];
// execute query...