How to lock mysql tables in php

Question

How do I lock mysql tables in php? I currently have this code:

$db->query("LOCK TABLES tbl_othercharge WRITE");
for($x=0;$x<=500; $x++){
    $id = get_max();
    $db->query("INSERT INTO tbl_othercharge SET tblocID = '$id', assessmentID='lock1'");
}

$db->query("UNLOCK TABLES");

Here's the get_max() function, which obviously will fail if the script above executes simultaneously.

 <?php
    function get_max(){
        global $db;
        $max = $db->get_var("SELECT MAX(tblocNumber) FROM tbl_othercharge");
        if($max == null){
            $max = 1;
        }else if($max >= 1){
            $max = $max + 1;
        }
        return 'OC'.$max;
    }
    ?>

I'm trying to test if there are still concurrency problems by executing the same script on 2 browsers. The script above inserts 400+ records instead of 999 records. How do I properly lock the table while I'm inserting something into it.

I want to lock the table to prevent something like this to happen:

As you can see the field with the prefix 'OC' on it should have a number which is equal to the auto-increment primary key.

Answer 1

The only reliable solution is to do an insert with a dummy value, getting the last insert id, and updating the row to the correct value.

mysql_query("INSERT INTO table (field) VALUES (dummy);");
$id = mysql_last_insert_id();
mysql_query("UPDATE table SET field='OC{$id}' WHERE id={$id} LIMIT 1;");