how to take difference of two date datatype variables

[fsl4faisal]

if($_POST['Submit']=="Check"){

        $issueid=(int)$_POST['issueid'];
        $returndate=mysql_real_escape_string($_POST['returndate']);

        list($temp)=
                    mysql_fetch_row(mysql_query("select issueid FROM issue where issueid='$issueid'"));
         if($temp!=NULL){

        $insert_query="insert into returning values($issueid,'$returndate')";
        $result=mysql_query($insert_query,$linkID1);

        if($result){
            list($bookid,$account,$issuedate,$duedate)=
                    mysql_fetch_row(mysql_query("select bookid,account,issuedate,duedate FROM issue where issueid='$issueid'"));
            list($title,$name,$fine)=
                    mysql_fetch_row(mysql_query("select title,name,fine FROM book,user where bookid='$bookid' AND acc='$account'"));

            $insert_book="update book set copies=copies+1 where bookid='$bookid'";
            $result=mysql_query($update_book,$linkID1);

    

 

 

i want to take difference of

 

$duedate and $returndate

 

and store it to another variable 

 

checks that variable for number of days in it