Generate random string in php without repetition

I'm trying to write a PHP function which will generate "supposedly" random strings which need to be unique regardless of the number of times it is run. Well, it can run more than once in order to generate but preferably not many many times.

For example, when you upload an image to imgur, it generates a random 5-letter [a-zA-Z] string. If I want to duplicate this (store the string with a Unique KEY in a MySQL database), without having to repeating select and ensure that the key does not already exist is there any way? At the same time, the importance of random does exist, so I'd rather not go 1,2,3 (aaaaa,aaaab,aaaac) as this would completely nullify the need for randomness.

I know there is 52^5 different possibilities but just for educational purposes (and future algorithm writing), is there an efficient method to generate these "unique" "random" strings?

[EDIT] I understand that unique+random is (basically) impossible. But is there any way I can generate unique, non-obvious strings? Thanks danishgoel!

Generate a random, unique, alpha-numeric string using PHP. Examples:

EA070
aBX32gTf

APPROACH 1: Brute Force The first approach is the simplest one to understand and thus brute force. 

It can be achieved as follows:

• Store all the possible letters into a string.
• Generate random index from 0 to string length-1.
• Print the letter at that index.
• Perform this step n times (where n is the length of string required).

Program: 

php

$n=10;

function getName($n) {

    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

    $randomString = '';

    for ($i = 0; $i < $n; $i++) {

        $index = rand(0, strlen($characters) - 1);

        $randomString .= $characters[$index];

    }

    return $randomString;

}

echo getName($n);

?>

APPROACH 2: Using Hashing Functions 

PHP has a few functions like md5(), sha1() and hash(), that can be used to hash a string based on certain algorithms like “sha1”, “sha256”, “md5” etc. All these function takes a string as an argument and output an Alpha-Numeric hashed string. 

To learn more about these functions click here. Once we understand how we utilize these functions, our task becomes pretty simple.

• Generate a random number using rand() function.
• Hash it using one of the above functions.

Program 1: 

php

$str=rand();

$result = md5($str);

echo $result;

7190bba9f6361764d423317d202402d5

7898decff6889ba2521bb32259e571be9880da25

690a4fea15d64168b512ad893f5e44bf13741a5c954f70fb6553e508965ad6f5

67de19022831ea109df3ab3b483a2c0912005fdc