php array_merge and array_intersect usage to merge, filter an array

Question

i need this a lot and i was thinking there must be a way to avoid looping arrays to accomplish this task.

example arrays

$array1 = ['user_id'=>'1','password'=>'PASS','name'=>'joe','age'=>'12'];
$array2 = ['user_id'=>'0','password'=>'default','age'=>'21'];
$filter = ['user_id','password'];

Question is how can i

1. merge array1 and array2 overwriting array2 with values of array1, and adding missing keys.

2. merge $array1 with $array2 overwritting $array2 with values from $array1, yet neglect extra data in array1 (above example should neglect name)

3. how can i unset all array_keys from $array1 which is in $filter

4. how to only return part of the array from $array1 where keys exist in $filter

without using loops ?

sorry if i ask for alot, but this is meant to collect most usages of array_intersect , array_merge, and array_diff and how to use them correctly.

edit:

Expected output

for 1.

    ['user_id'=>'1','password'=>'PASS','name'=>'joe','age'=>'12']; //since all array2 was overwritten and extra keys was added

2.

    ['user_id'=>'1','password'=>'PASS','age'=>'12'];

3. 

    ['age'=>'21']; //removed user_id,password from array1 since they exist in $filter

4. 

    ['user_id'=>'1','password'=>'PASS','age'=>'12'];//return only values of keys that exist in $filter

thanks

Answer 1

1-merge array1 and array2 overwriting array2 with values of array1, and adding missing keys.

$a1 = $a1 + $a2;

2-merge $array1 with $array2 overwritting $array2 with values from $array1, yet neglect extra data in array1 (above example should neglect name)

$a2 = $a2 + $a1;

3-how can i unset all array_keys from $array1 which is in $filter

array_walk($array1, function($val,$key) use(&$array1, $filter) { 
    if(in_array($key, $filter)) unset($array1[$key]);
});

4-how to only return part of the array from $array1 where keys exist in $filter

array_walk($array1, function($val,$key) use(&$array1, $filter) { 
    if(!in_array($key, $filter)) unset($array1[$key]);
});