<?php
$new_prefix = 'new_';
require_once 'configuration.php';
$config = new JConfig;
$con = mysql_connect($config->host, $config->user, $config->password);
if(!is_resource($con)) die('Error connecting to db');
$test = mysql_select_db($config->db, $con);
if($test===false) die('Error connecting to db');
$prefix = $config->dbprefix;
$sql = "show tables where `Tables_in_{$config->db}` like '{$prefix}%'";
$res = mysql_query($sql);
while($row = mysql_fetch_array($res))
{
    $old = $row[0];
    $new = $new_prefix . substr($old, 4);
    $temp = mysql_query("RENAME TABLE `$old` TO `$new`");
    if($temp === false) die(mysql_error());
    mysql_free_result($temp);
}
mysql_free_result($res);
mysql_close($con);
echo "OK";
?>

http://magazine.joomla.org/topics/item/108-the-prefix-has-nothing-to-do-with-telephony

Substitute new_ placeholder in the second line with your new prefix.  Run it: http://www.yoursite.com/rename.php and wait until it responds with an OK, usually after a couple of seconds.  Delete the rename.php script.  Edit the configuration.php file and find the line starting with var $dbprefix. It should look like this:  var $dbprefix = \'jos_\';  Replace the old jos_ prefix with new prefix, i.e. the one used in the second line of your rename.php script.

Change Default Joomla Database Table Prefix

mysql, php, security, joomla

PHP