Question: I have to generate a list of all possible combinations of numbers and letters of length three. The catch is on the first two characters can be letters or numbers, and the third one can only be numeric.

For example:

```
AA1, AA2, AA3 .... FC7 ... 001, 002 ... 365)

```

I hope you can all help me. I look forward to these responses. Regards, Josh.

So far i only managed a very simple way to get all number with prevailing zeros 

```
for ($k = 0 ; $k < 999; $k++) {
     $rnd[] = sprintf('%03d',$k);
}

```

Answer: 
This should work for you:

Basically I have an array with all letters (`[A-Z]`) and an array with all numbers (`[0-9]`). Then you define which order of possible characters you want. E.g. here you want `letterNumber`, `letterNumber` and then the third spot only `number`.

After this you loop through everything as many times as characters per combinations which you want(e.g. `XXX -> 3 times`). In the loop you go through all combinations which you already have with all characters which you want at this spot.

So after the 1 iteration you have an array with the first character of each combination, which would be: `[0-9A-Z]`.

Then in the second iteration you go through all combinations which you already have, here `[0-9A-Z]` with the characters which you want on the second spot, here `[0-9A-Z]`. So for ever character in the combinations array ([`0-9A-Z`]) you get a new combinations with each character of `[0-9A-Z]`.

And this repeated over and over until you get your expected combination length.

So at the end you end up with:

```
letterNumber = 36 = 26 + 10 possible characters (**[A-Z0-9]**)
letter = 26 possible characters (**[A-Z]**)
number = 10 possible characters (**[0-9]**)

```

> 
> 36 \* 36 \* 10 = 12'960 combinations
> 
> 
> 

The code:

```
<?php

    $letters = range("A", "Z");
    $numbers = array_merge(range(0, 9));
    $order = ["letterNumber", "letterNumber", "number"]; //possibilities: "letter", "number", "letterNumber"

    $length = count($order);
    $combinations = [[]];

    for($count = 0; $count < $length; $count++) {
        $tmp = [];

        if($order[$count] == "number" || $order[$count] == "letterNumber") {
            foreach($combinations as $combination) {
                foreach($numbers as $v)
                    $tmp[] = array_merge($combination, [$v]);
            } 
        }
        if($order[$count] == "letter" || $order[$count] == "letterNumber") {
            foreach($combinations as $combination) {
                foreach($letters as $v)
                    $tmp[] = array_merge($combination, [$v]);
            }
        }

        $combinations = $tmp;

    }

    print_r($combinations);

?>

```

output:

```
Array
(
    [0] => Array
        (
            [0] => 0
            [1] => 0
            [2] => 0
        )

    [1] => Array
        (
            [0] => 0
            [1] => 0
            [2] => 1
        )
    //...

    [12959] => Array
        (
            [0] => Z
            [1] => Z
            [2] => 9
        )

)

```

[`**Demo**`](http://viper-7.com/zuo74A)
The following code worked perfectly for what i was after.

```
$letters = range('A', 'Z');
$comb = array();
for ($k = 0 ; $k <= 9; $k++) {
    foreach($letters as $l){
        foreach($letters as $le){
            $comb[]= $l.$le.$k;
        }
    }
}
for ($k = 0 ; $k <= 999; $k++) {
    $comb[] = sprintf('%03d',$k);
}

```