Simple key white-list filter:

<?php
$arr = array('a' => 123, 'b' => 213, 'c' => 321);
$allowed = array('b', 'c');

print_r(array_intersect_key($arr, array_flip($allowed)));
?>

Will return:
Array
(
    [b] => 213
    [c] => 321
)

Note that the order of the keys in the returned array is the same as the order of the keys in the source array.

To sort by the second array, then you may do so through array_replace.

<?php
$array = array(
    'two'   => 'a',
    'three' => 'b',
    'one'   => 'c',
    );

$keyswant = array(
    'one'       => '',
    'three'     => '',
    );

print_r(array_intersect_key(array_replace($keyswant, $array), $keyswant));

?>

Shows:

Array
(
    [one] => c
    [three] => b
)

Rather than:

Array
(
    [three] => b
    [one] => c
)

[Editor's note: changed array_merge_recursive() to array_replace_recursive() to fix the script]

Here is a better way to merge settings using some defaults as a whitelist.

<?php

$defaults = [
    'id'            => 123456,
    'client_id'     => null,
    'client_secret' => null,
    'options'       => [
        'trusted' => false,
        'active'  => false
    ]
];

$options = [
    'client_id'       => 789,
    'client_secret'   => '5ebe2294ecd0e0f08eab7690d2a6ee69',
    'client_password' => '5f4dcc3b5aa765d61d8327deb882cf99', // ignored
    'client_name'     => 'IGNORED',                          // ignored
    'options'         => [
        'active' => true
    ]
];

var_dump(
    array_replace_recursive($defaults, 
        array_intersect_key(
            $options, $defaults
        )
    )
);

?>

Output:

array (size=4)
    'id'            => int 123456
    'client_id'     => int 789
    'client_secret' => string '5ebe2294ecd0e0f08eab7690d2a6ee69' (length=32)
    'options'       => 
        array (size=2)
            'trusted' => boolean false
            'active'  => boolean true

Note that the order of the keys in the returned array is the same as the order of the keys in the source array. eg:

<?php
$array = array(
    'two'   => 'a',
    'three' => 'b',
    'one'   => 'c',
    );

$keyswant = array(
    'one'       => '',
    'three'     => '',
    );

print_r(array_intersect_key($array, $keyswant));

?>

Shows:

Array
(
    [three] => b
    [one] => c
)

If you want an array that has no key value pairs added from the second array:

$new = array_intersect_key($b, $a) + $a;

I have found the following helpful:
<?PHP
function array_merge_default($default, $data) {
        $intersect = array_intersect_key($data, $default); //Get data for which a default exists
        $diff = array_diff_key($default, $data); //Get defaults which are not present in data
        return $diff + $intersect; //Arrays have different keys, return the union of the two
}
?>
It's use is like both of the functions it uses, but keeps defaults and _only_ defaults. It's designed for key arrays, and i'm not sure how it will work on numeric indexed arrays.

Example:
<?PHP
$default = array(
 "one" => 1,
 "two" => 2
);
$untrusted = array(
 "one" => 42,
 "three" => 3
);
var_dump(array_merge_default($default, $untrusted));

array(2) {
  ["two"]=>
  int(2)
  ["one"]=>
  int(42)
}

?>

Jesse: no, array_intersect_key does not accomplish the same thing as what you posted:

array_flip (array_intersect (array_flip ($a), array_flip ($b)))

because when the array is flipped, values become keys. having duplicate values is not a problem, but having duplicate keys is. array_flip resolves it by keeping only one of the duplicates and discarding the rest. by the time you start intersecting, you've already lost information.

Regarding php at keithtylerdotcom solution to emulate

<?php
$z = someFuncReturningAnArray()['some_key'];
?>

His recommended solution will still return an array. To get the value of a single key in an array returned by a function, simply add implode() to the recipe:

<?php
function someFuncReturningAnArray() {
  return array(
    'a' => 'b',
    'c' => 'd',
    'e' => 'f',
    'g' => 'h',
    'i' => 'j'
  );
}

//traditional way
$temp = someFuncReturningAnArray();
$b = $temp['a'];
echo print_r($b, 1) . "\n----------\n";

//keithtylerdotcom one-line method
$b = array_intersect_key(someFuncReturningAnArray(), array('a'=>''));
echo print_r($b, 1) . "\n----------\n";

//better one line method
$b = implode('', array_intersect_key(someFuncReturningAnArray(), array('a'=>'')));
echo print_r($b, 1) . "\n----------\n";
?>

<?php
    /**
     * calculates intersection of two arrays like array_intersect_key but recursive
     *
     * @param  array/mixed  master array
     * @param  array        array that has the keys which should be kept in the master array
     * @return array/mixed  cleand master array
     */
    function myIntersect($master, $mask) {
        if (!is_array($master)) { return $master; }
        foreach ($master as $k=>$v) {
            if (!isset($mask[$k])) { unset ($master[$k]); continue; } // remove value from $master if the key is not present in $mask
            if (is_array($mask[$k])) { $master[$k] = $this->myIntersect($master[$k], $mask[$k]); } // recurse when mask is an array
            // else simply keep value
        }
        return $master;
    }
?>

in case you came here looking for a function that returns an array containing the values of `all` arrays with intersecting keys:
<?php
   function array_merge_on_key($key, $array1, $array2) {
      $arrays = array_slice(func_get_args(), 1);
      $r = array();
      foreach($arrays as &$a) {
         if(array_key_exists($key, $a)) {
            $r[] = $a[$key];
            continue;
         }
      }
      return $r;
   }
   // example:
   $array1 = array("id" => 12, "name" => "Karl");
   $array2 = array("id" => 4, "name" => "Franz");
   $array3 = array("id" => 9, "name" => "Helmut");
   $array4 = array("id" => 10, "name" => "Kurt");

   $result = array_merge_on_key("id", $array1, $array2, $array3, $array4);

   echo implode(",", $result); // => 12,4,9,10
?>

Just a simple script if you want to use one array, which contains only zeros and ones, as mask for another one (both arrays must have the same size of course). $outcome is an array that contains only those values from $source where $mask is equal to 1.

<?php
$outcome = array_values(array_intersect_key( array_values($source), array_filter(array_values($mask)) ));
?>

PS: the array_values() function is necessary to ensure that both arrays have the same numbering/keys, otherwise your masking does not behave as you expect.

Enjoy!