php unique username.php

<?php 

function generateUsername() {
    $small = array('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z');
    $capital = array('A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z');

    $numbers = array(0, 1, 2, 3, 4, 5, 6, 7, 8, 9);

    $f = rand(0, 25);
    $s = rand(0, 25);

    $firstPart = $small[$f] . $small[$s];
    $secondPart = $capital[$f] . $capital[$s];

    if (($f > 0 && $f < 10) && ($s > 0 && $s < 10)) {
        $thirdPart = $numbers[$f] . $numbers[$s];
    } else {
        $f = rand(0, 9);
        $s = rand(0, 9);
        $thirdPart = $numbers[$f] . $numbers[$s];
    }

    return str_shuffle($firstPart . $secondPart . $thirdPart);
}

$sum = 0;
for ($i = 0; $i < 5000; $i++) {
    ob_start();
    $start = microtime();
    echo generateUsername();

    $end = microtime();
    ob_end_clean();
//
    $time = ($end - $start);
    $sum += $time;
}

echo 'Total avg execution time : ' . ($sum/5000) . ' seconds';

?>

Total execution time : 0.00027600000000005 seconds
Total execution time : 0.000524 seconds
Total execution time : 0.00027699999999997 seconds
Total execution time : 0.000274 seconds
Total execution time : 0.00027500000000003 seconds

<?php
 function genUserName($length=6) {
$pool = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'; // add other character you want
return substr(str_shuffle(str_repeat($pool, 6)), 0, $length);
} 
?>

Easy to understand And Simple .... :P Hope It Works better For you Bro :)