function four_letters_combination()
{
$s = "abcdefghijklmnopqrstuvwxyz";
$s_size = strlen($s);
for ($i = 0; $i < $s_size; $i++)
{
for ($j = 0; $j < $s_size; $j++)
{
for ($k = 0; $k < $s_size; $k++)
{
for ($m = 0; $m < $s_size; $m++)
{
echo $s[$i] . $s[$j] . $s[$k] . $s[$m] . " ";
}
}
}
echo "<br />";
}
}
four_letters_combination();
/*
run:
aaaa aaab aaac aaad aaae aaaf aaag aaah aaai aaaj aaak aaal aaam aaan aaao aaap aaaq aaar ...
aaaz aaba aabb aabc aabd aabe aabf aabg aabh aabi aabj aabk aabl aabm aabn aabo aabp aabq ...
aaby aabz ...
...
zzxw zzxx zzxy zzxz zzya zzyb zzyc zzyd zzye zzyf zzyg zzyh zzyi zzyj zzyk zzyl zzym zzyn ...
zzyv zzyw zzyx zzyy zzyz zzza zzzb zzzc zzzd zzze zzzf zzzg zzzh zzzi zzzj zzzk zzzl zzzm ...
zzzu zzzv zzzw zzzx zzzy zzzz
*/
