generate_unique_key.php

    function generate_unique_key
    (
        $column_name = null,
        $db = null,
        $table_name = null,
        $size = null
    )
    {
        if(is_null($size) || is_null($column_name) || is_null($table_name) || is_null($db))
        {
            return null;
        }
        else
        {
            $unique_key = "";
            $code = "";
            // we are creating unique value with rand function
            // rand function http://php.net/manual/en/function.rand.php
            for($i=0;$i<$size;$i++) {
                $code = $code . rand(0,9);
            }
            // here we will check if key is unique
            // please refer this video, code is explained here https://www.youtube.com/watch?v=MCz-s06Pzo0
            if(
                is_unique
                (
                    $column_name,
                    $db,
                    $table_name,
                    $code
                )
            )
            {
                // if that value is unqiue, we will return it 
                return $code;
            }
            else
            {
                // if that value is not unique, we will create it again
                generate_unique_key
                (
                    $column_name,
                    $db,
                    $table_name,
                    $size
                );
            }
        }