Generate Unique number in php and mysql

There are number of way to generate unique number or id in mysql  and  php.

Using Rand() function. we can get random data with in  limit

SELECT  *   FROM company ORDER BY RAND() LIMIT 10


<?
 echo str_replace ("}","",str_replace("{","",com_create_guid()));
?>
<?$number = mt_rand(500 pan>,1000); ?>

<? $uniqid = date('U') . mt_rand(0,9); ?>

<? echo time();?>