PHP UniqueID:
Generate unique ID in PHP
A unique ID can be created in PHP using the uniqid () function.

uniqid():Returns the unique identifier, as a string.
Here are some examples:

<?php 
 //creates a unique id with the 'unique' prefix
 $a = uniqid('unique');
 echo $a;
 echo "<br>";
 //creates a longer unique id with the 'unique' prefix
 $b = uniqid ('unique', true);
echo $b;
 echo "<br>";
 //creates a unique ID with a random number as a prefix
 $c = uniqid (rand (),true);
 echo $c;
 echo "<br>";
 //this md5 encrypts the username from above
 $md5c = md5($c);
 echo $md5c;
 ?>