回到给定索引的值

$a = array('a' => 'apple', 'b' => 'banana', 'p' => 'pear');
$key = array('a','b','p');
foreach($key as $k) $val[] = $a[$k];

print_r($val);

Array
(
    [0] => apple
    [1] => banana
    [2] => pear
)

$a = array(
  'a'=>'apple',
  'b'=>'banana',
  'p'=>'pear'
);

$key = array('a','b','p');

$val = array();

foreach($key as $v){
    array_push($val, $a[$v]);
}

print_r($val);