Question

I would like to do something similar to the following.

$ind=array('a','b','c');
$arr['a']=1;
$arr['b']=4;
$arr['c']=7;
$d= $arr[$ind];
print_r($d);

It obviously doesnt work and I was wondering if there is still a simple way to do it in 1 or two lines of code.

EDIT

Here is a better example to demonstrate what I mean.

$ind=array('a','c','b','d','e','a','a');
$arr['a']=1;
$arr['b']=4;
$arr['c']=7;
$d=$arr[$ind];

Now I want to receive that $d is equal to 1,7,4,1,1

array_intersect_key doesnt work. Or at least I cant make it work for me. I was thinking array_walk and a lamda function or something. Anyone?

NOTE: This syntax is available in Matlab (well, almost). So you could for example type:

arr=rand(20,1);
ind=[1 2 10 2 1 1];
newArr=arr(ind); 

and it would produce vector of length 6 with the values as indexed by the ind vector.

Answer 1

To get the items from $arr which have keys matching the values in $ind, you can use the following:

$arr = array('a' => 1, 'b' => 4, 'c' => 7);
$ind = array('a','c');
$d   = array_intersect_key($arr, array_flip($ind));
// $d = array('a' => 1, 'c' => 7);

The call to array_flip() simply “flips” the keys/values in $ind (to make it look like array('a'=>0,'c'=>1)) then we find the keys intersection with array_intersect_key() which returns the items from the first array ($arr) which have keys matching the second array (the flipped $ind).

---

Edit Building on mario’s answer (which is broken at the time of writing, and doesn’t do what you appear to want) you could use

class MultiIndex extends ArrayObject
{
    public function offsetGet($offset)
    {
        if (is_array($offset)) {
            $return = array();
            foreach ($offset as $o) {
                if ($this->offsetExists($o)) {
                    $return[] = parent::offsetGet($o);
                }
            }
            return $return;
        } else {
            return parent::offsetGet($offset);
        }
    }
}

$arr = new MultiIndex($arr);
$d = $arr[$ind];
// $d = array(1,7,4,1,1);