multidimensional array to single array
$array=[1=>[2,5=>[4,2],[7,8=>[3,6]],5],4];
$arr=[];
array_walk_recursive($array, function($k){global $arr; $arr[]=$k;});
print_r($arr);
Output
Array ( [0] => 2 [1] => 4 [2] => 2 [3] => 7 [4] => 3 [5] => 6 [6] => 5 [7] => 4 )