Changing the database table prefix in Wordpress does NOT increase security.

prefix.php

<?php

/**
 * This code may not work, it's pretty much pseudocode, as I'm presuming get_results() will return an array of tables. I don't have a Wordpress installation to test it!
 * The idea is correct though, and the command `SHOW TABLES` will 100% return a list of tables in the database.
 * 
 * But for arguments sake, lets assume the format returned is as below.
 */
 
function getWordpressTablePrefix() {
    $tables = $wpdb->get_results("SHOW TABLES");
    
    foreach ($tables as $table) {
        $output = array();
        
        if (preg_match("/(.*)_users/i", $table, $output)) {
            return $output[1];
        }
    }
}

/**
 * Calling `getWordpressTablePrefix()` *will* return the prefix used by this installation (for example, 'wp' or 'more_secure_wp'). 
 * 
 * It is dangerous to assume changing the table prefix will keep you safe from anybody that's achieved database injection.
 */