$numplate = "";
$chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
for ($i = 0; $i == 3; $i++)
{
    $pos = rand(0, strlen($chars)-1);
    $numplate .= $chars{$pos};
    echo "$numplate";
}
$chars2 = "0123456789";
for ($i = 0; $i == 3; $i++)
{
    $pos2 = rand(0, strlen($chars2)-1);
    $numplate .= $chars2{$pos2};
}

echo "<br>Numberplate is $numplate<br>";