<?php
$chars = "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";
$chars2 = "0,1,2,3,4,5,6,7,8,9";
$arrayChars = explode("," , $chars);
$arrayNumbers = explode("," , $chars2);
$result = array_merge(array_rand(array_flip($arrayChars) , 3) , array_rand(array_flip($arrayNumbers) , 3));
shuffle($result);
var_dump($result);
preferences:
22.67 ms | 409 KiB | 5 Q