无法将生成的随机ID插入数据库

I am trying to get the value of the random id from another page through session, but while inserting into the database it doesn’t get inserted into database. It’s zero, here is the code:

Stud.php
     <?php  
                session_start();
                $seed = str_split('abcdefghijklmnopqrstuvwxyz1234567890');
                $rand = array_rand($seed, 7);
                $convert = array_map(function($n){
                global $seed;
                return $seed[$n];
                },$rand);
                $var = implode('',$convert);
                echo $var;
                //echo "<blink>Your Complaint is is:".$var."</blink>";
                // $_SESSION['varname'] = $var;
                // unset($comment);

            ?>

Insert.php

<?php
session_start();
include 'library/dbconnect.php';
$_SESSION['varname'] = $var;
$var_value = $_SESSION['varname'];$name=$_POST['Studname'];
$district=$_POST['district'];
$school_name=$_POST['School_name'];
$standard=$_POST['std'];
$division=$_POST['Division'];
$complaint=$_POST['Complaint'];
$entryDate=$_POST['date'];
$dateEntry=explode('-',$entryDate);
$date1=$dateEntry[2]."-".$dateEntry[1]."-".$dateEntry[0];
echo $name."<br />".$district."<br />".$date1."<br />";
$query2="INSERT INTO `Complaint_register`(`complain_Id`,`entrydate`, `studname`,
`District`, `School_name`, `Standard`, `Division`, `Complaint`, `status`) VALUES 
('$var_value','$date1','$name','$district','$school_name','$standard',
'$division','$complaint',1)";   
$result2=mysql_query($query2)or die(mysql_error());
$query3="SELECT * FROM Complaint_register";
$result3=mysql_query($query3);
$num=mysql_numrows($result3);
 if($result2)
{
$coment= $num.' Complaint Successfully Registered';
header("Location:stud.php");

}
?>

check the type of : complain_Id change it to varchar(1024)