Form session not working when multiple users use it - php

Question

i need an help from you all.
i had created an form using PHP. it's a school application registration form. it has one page only.
i need to generate registration number(session id used as registration number here) for everyone who opens the form.
instead of creating a session i have used ID for all. that is when some one submits the form, it checks the DB and if the registration number is there, it will increment one value and add the current form to DB.
my code here
<td>Application No : <input type="hidden" name="disablusr_dummyid" autocomplete="off" style="background:#f0efed;" value="00<?php
include('config.php');
$q="select MAX(auto_gen_id) from application_form";
$result=mysql_query($q);
$data=mysql_fetch_array($result);
$max_val=$data[0];
echo $max_val+1;
?>"/>
<input type="hidden" name="applicant_id" autocomplete="off" value="00<?php
include('config.php');
$wer = "select MAX(auto_gen_id) from application_form";
$resultgh = mysql_query($wer);
$dates = mysql_fetch_array($resultgh);
$erfdqwe = $dates[0];
echo $erfdqwe+1;
?>" />
<input type="hidden" name="txt_applicant_id" style="display:none;" autocomplete="off" value="<?php
include('config.php');
$werqw = "select MAX(auto_gen_id) from application_form";
$resultghasw = mysql_query($werqw);
$dataqsax = mysql_fetch_array($resultghasw);
$erfdqweqti = $dataqsax[0];
echo $erfdqweqti+1;
?>" /></td>
but what is happening is when multiple users are using the form same session ID is generated and only one user is able to save the form and it reflects in DB. other forms are being not submitted and not added to DB.
help me in this error.. thanks in advance.

Answer 1

The solution here is not the use the hidden fields for the IDs :
At this moment u have something like this :
"INSERT INTO (id, ..., ...) VALUES('".addslashes($_POST['applicant_id']."', ...)
You should refactor your logic to calculate the id afterwards meaning that u drop the hidden fields and do this :
<?php
if (!empty($_POST)) {
$wer = "select MAX(auto_gen_id) from application_form";
$resultgh = mysql_query($wer);
$dates = mysql_fetch_array($resultgh);
$erfdqwe = $dates[0];
$new_applicant_id = $erfdqwe+1;
}
And then use the $new_applicant_id into the INSERT sql.
On a sidenote your code is vulnerable for SQL injection and is using outdated functions.
You should try to move to mysqli or PDO and preferable use prepared statements