How to filter an associative arrays using array of keys in PHP?

Question

I have an associative arrays and an array of keys.

$A = array('a'=>'book', 'b'=>'pencil', 'c'=>'pen');
$B = array('a', 'b');

How I build an associative array from all element of $A where the key is in $B? For the example above, the answer should be

$C = array('a'=>'book', 'b'=>'pencil');

Answer 1

$keys = array_flip($B);
$C = array_intersect_key($A,$keys);

Answer 2

array_intersect_key($A,array_combine($B,$B))

or better: array_intersect_key($my_array, array_flip($allowed))

from the question: PHP: How to use array_filter() to filter array keys?