mixed array to string via recursive function

When I loop through the given array, it suddenly sets back to the beginning:

Here it starts in a classes method:

$where = 'WHERE ';
array_walk( $columns, array( &$this, 'walker_cols' ), $where );

And this is the callback

function walker_cols( $item, $key, &$where )
{
    static $temp;
    if ( is_array( $item ) )
    {
        $temp = $key;
        array_walk( $item, array( __CLASS__, __FUNCTION__ ), $where );
    }

    var_dump($item);
    var_dump($where);

    return $where .= $temp ? "{$temp} = {$item} AND " : "{$key} = {$item} AND ";
}

The given Array:

$columns = array(
     'assoc_key_a' => 'assoc_val_a'
    ,'assoc_key_b' => array(
         0 => 'num_val_a'
        ,1 => 'num_val_b' 
     )
);

The desired output:

WHERE assoc_key_a = assoc_val_a AND assoc_key_b = num_val_a AND assoc_key_b = num_val_b

Now my result from the var_dump is the following:

input: "assoc_val_a"
output: "WHERE "

input: "num_val_a"
output: "WHERE assoc_key_a = assoc_val_a AND "

input: "num_val_b"
output: "WHERE assoc_key_a = assoc_val_a AND assoc_key_b = num_val_a AND "

input: {
    [0] => "num_val_a"
    [1] => "num_val_b"
}
output: "WHERE assoc_key_a = assoc_val_a     AND "

If there's another way to come to the desired output, I'd be happy to walk it. I already tried to do it with array_walk_recursive(), but with that function I wasn't able to get the assoc_key_b, because it directly jumped into the sub-array.

Thanks for any help. I'm pretty stuck.