How to generate unique 6 digit code

I want to generate 6 digit unique code but i want first 3 are alphabets and last 3 are numbers like below example..

AAA111
ABD156
DFG589
ERF542...

Please help to create code with above combinations..

below is my code..

public function generateRandomString()  {
        $characters = '1234567890';
        $length = 6;
        $charactersLength = strlen($characters);
        $randomString = '';
        for ($i = 0; $i < $length; $i++) {
            $randomString .= $characters[rand(0, $charactersLength - 1)];
        }
        return $randomString;
    }