如果键名不存在，array_column会返回什么？

Question

According to https://wiki.php.net/rfc/array_column array_column is slated to be added to PHP soon. But I having trouble understanding the RFC. What will be returned if a named key doesn't exist?

Example:

$arr = array(
    array(
        'firstname' => 'Bob',
        'lastname'  => 'Tomato'
    ),
    array(
        'firstname' => 'Larry',
        'lastname'  => 'Cucumber'
    )
);

$middlenames = array_column($arr, 'middlename');

Answer 1

Introduction

For you to understand the RFC you need to understand the problem first and the reason it was introduced.

Your Array

$arr = array(
        array(
                'firstname' => 'Bob',    <--
                'lastname' => 'Tomato'     |    <--
        ),                                 |      |
        array(                             |      |
                'firstname' => 'Larry',  <--      |
                'lastname' => 'Cucumber'        <-|
        )
);

Getting Column

To get Bob & Larry or Tomato and Cucumber you have use more than one line of code examples are :

$colums = array();
foreach ( array_map(null, $arr[0], $arr[1]) as $value ) {
    $colums[] = $value;
}
print_r($colums);

Output

Array
(
    [0] => Array
        (
            [0] => Bob
            [1] => Larry
        )

    [1] => Array
        (
            [0] => Tomato
            [1] => Cucumber
        )

)

Dynamic Version

The code above would only work if you know number of elements another creative way would be

$colums = array();
array_unshift($arr, null);
foreach (call_user_func_array("array_map", $arr) as $value ) {
    $colums[] = $value;
}

Live Test

Or Better Sill use MultipleIterator

$mi = new MultipleIterator(MultipleIterator::MIT_NEED_ALL);
foreach ( $arr as $v ) {
    $mi->attachIterator(new ArrayIterator($v));
}

$colums = array();
foreach ( $mi as $v ) {
    $colums[] = $v;
}
print_r($colums);

Live Test

Key Name

If you need to get the key name here is another method

$colums = array_reduce($arr, function ($a, $b) {
    foreach ( $b as $k => $v ) {
        $a[$k][] = $v;
    }
    return $a;
});

Live Test

Back to array_column

array_column intends simply the process and getting all columns with first name would be as simple as the following:

 print_r(array_column($arr,"lastname"));
                               ^
                               |
                               +--- This get value with key "lastname"

Live Test

More Complex Senerio

Imagine you want your array to have this output

Array
(
    [Bob] => Tomato
    [Larry] => Cucumber
)

Use Old methods you can have

$colums = array();
array_unshift($arr, null);
foreach (call_user_func_array("array_map", $arr) as $value ) {
    $key = array_shift($value);
    $colums[$key] = current($value);
}
print_r($colums);

Live Test

Now you can see i had to use array_shift and current to get first 2 element .. as your array grows this can become complex but array_column would simplify this

print_r(array_column($arr,"lastname","firstname"));
                              ^           ^
                              |           |
                             Value       Key    (I wonder why position is backwards)

Output

Array
(
    [Bob] => Tomato
    [Larry] => Cucumber
)

Finally Back to your Question

What will be returned if a named key doesn't exist?

Empty array ... From your example

 print_r(array_column($arr,"middlename"));
                                ^
                                |
                        it would try to check if any of your array has key middle man

It returns

Array        <------- Otherwise returns empty array 
(
)

Conclusion

I used so may different examples using loop , array_map , array_reduce and MultipleIterator to explain what array_column is trying to achieve.

As you can see array_column is much more simplified but i would advice you play with the examples in the RFC a little and this would allow you understand it better if you still don't understand it, PHP is a flexible language you can always implement your own version