Question

I’d like to know if there is any native way to extract a set of values and their keys from a large array using a smaller array of keynames.

compact() does this on separate variables but not on arrays:

$foo='foo';
$bar='bar';

print_r(compact('foo','bar'));
// or
print_r(compact(array('foo','bar'));

// Prints:
// Array(
//  [0] => foo,
//  [1] => bar
// )

So far I’ve always used foreach loops, but I started to wonder if a simpler method exists. I’ve gone through the PHP documentation’s Array functions section but still felt that I’d try asking in case I missed something.

Answer 1

To summarise what turned out more lengthy than I anticipated, the functions of interest to you are likely array_flip() and array_intersect_key().

---

I hope I correctly understood what you wanted. You want to an array of variable names, where the variables themselves contain array keys, and you want to search a larger array for these keys?

Does this do what you want? I haven’t benchmarked it against a foreach, it’s quite possibly slower.

$foo = 'foo2';
$bar = 'bar3';

$array = array(
    'foo1' => 1,
    'foo2' => 2,
    'foo3' => 3,
    'foo4' => 4,
    'bar1' => 5,
    'bar2' => 6,
    'bar3' => 7,
    'bar4' => 8
);

$keys = array(
    'foo',
    'bar'
);

$extract = array_intersect_key($array, array_flip(compact($keys)));

var_dump($extract);

Output:

array
  'foo2' => int 2
  'bar3' => int 7

Edit:
From Gordons comment, if the use of compact to use variable names as keys is irrelevant, you can substitute in the following code.

$keys = array(
    'foo2',
    'bar3'
);

$extract = array_intersect_key($array, array_flip($keys));

You can further reduce the code by defining your array of keys slightly differently, and eliminating the need for array_flip().

$keys = array(
    'foo2' => null,
    'bar3' => null
);

$extract = array_intersect_key($array, $keys);