How to combine 3-4 characters from the six characters in a-f?

{Code ...} take 3 or 4 characters from the array $ arr and combine them into new characters, such as abc, abd, abe, and abcd, when the order is different, abc and acb are considered different. New characters are stored in the array, $ newArr [] & #039; abc & #039 ;; $ newArr [] & #039; abd & #039; $ n...

$arr=['a','b','c','d','e','f'];

Take 3 or 4 characters from the array $ arr and combine them into new characters, such as abc, abd, abe, and abcd, when the order is different, abc and acb are considered different, and new characters are stored in the array,
$ NewArr [] = 'abc ';
$ NewArr [] = 'abd ';
$ NewArr [] = 'AB ';
$ NewArr [] = 'abcd ';
$ NewArr [] = 'acbd ';
...

How to list all the situations?

Reply content:

$arr=['a','b','c','d','e','f'];

Take 3 or 4 characters from the array $ arr and combine them into new characters, such as abc, abd, abe, and abcd, when the order is different, abc and acb are considered different, and new characters are stored in the array,
$ NewArr [] = 'abc ';
$ NewArr [] = 'abd ';
$ NewArr [] = 'AB ';
$ NewArr [] = 'abcd ';
$ NewArr [] = 'acbd ';
...

How to list all the situations?

function dfs($pre, $chars, $arr, $lenArr) {    if(!empty($pre) && in_array(strlen($pre), $lenArr)){ $arr[] = $pre; }    if(!empty($chars)) {        foreach ($chars as $char) {            $tempChars = array();            foreach ($chars as $c) {                if ($c !== $char) { $tempChars[] = $c; }            }            $arr = $this->dfs($pre.$char, $tempChars, $arr, $lenArr);        }    }    return $arr;}function get_combine() {    $chars = array('a', 'b', 'c', 'd', 'e', 'f');    $combineArray = array();    $combineArray = $this->dfs('', $chars, $combineArray, array(3, 4));    echo count($combineArray).'
';    var_dump($combineArray);}

Recursion can be used to achieve