Please help the master to modify a recursive function, so that it has a return value

Please help the master to modify the recursive function, so that it has a return value
At the moment I have a recursive function that echoes out the ID value in a multi-dimensional array
Echo to page appears as "1,2,3,4,5 ..."

PHP Code

  
   function Arr_fun ($arr) {        if (Is_array ($arr)) {          foreach ($arr as $v) {             if (Is_array ($v)) {             arr_fun ($v);             } else{                  echo $arr [id]. ",";                Break;}}}        else{             Echo $arr. ",";        }  

Because I need to use this string of ID strings from ECHO.
Attempt to assign value to $s unsuccessful $s =arr_fun ($array _menu); Note: $array _menu is a dynamic multidimensional array whose dimensions are dynamically changing
So I think the above method has no return value, so it can't be assigned
Hope that the elder enlighten, help me revise, how can let its return value also "1,2,3,4 ..."?


------Solution--------------------

PHP Code

function Arr_fun ($arr) {        $r = ';      if (Is_array ($arr)) {          foreach ($arr as $v) {             if (Is_array ($v)) {             $r. = Arr_fun ($v);             } else{                  $r. = $arr [id]. ",";                Break;}}}        else{             $r. = $arr. ",";        }      return $r;  
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