<?php

function gerarCobinacao($tam){
    $seed = str_split('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789');
    shuffle($seed); // probably optional since array_is randomized; this may be redundant
    $rand = '';
    foreach (array_rand($seed, $tam) as $k) 
      $rand .= $seed[$k];

    return $rand;
 }

print gerarCobinacao(5);