Метод 1: использование array_map() и Trim()

$array = array("  key1 " => "value1", "key2 " => "value2", " key3" => "value3");
$array = array_map('trim', array_combine(array_map('trim', array_keys($array)), $array));
print_r($array);

Выход:

Array
(
    [key1] => value1
    [key2] => value2
    [key3] => value3
)

Метод 2: использование array_reduce()

$array = array("  key1 " => "value1", "key2 " => "value2", " key3" => "value3");
$array = array_reduce(
    $array,
    function ($result, $value) {
        $key = trim(array_keys($result)[count($result)]);
        $result[$key] = $value;
        return $result;
    },
    array()
);
print_r($array);

Выход:

Array
(
    [key1] => value1
    [key2] => value2
    [key3] => value3
)

Метод 3. Использование цикла foreach

$array = array("  key1 " => "value1", "key2 " => "value2", " key3" => "value3");
$newArray = array();
foreach ($array as $key => $value) {
    $trimmedKey = trim($key);
    $newArray[$trimmedKey] = $value;
}
print_r($newArray);

Выход:

Array
(
    [key1] => value1
    [key2] => value2
    [key3] => value3
)