Why is the splat operator not considered an expression?

Question

I'm not sure if that title is phrased exactly right but my that is how my IDE (PHPStorm) describes it.

Using the splat operator, I want to take an array that looks like this:

array(
    array(
        'label' => 'Some label',
        'value' => 'some-value'
    )
)

And transform it into one that looks like this:

array(
    'some-value' => 'Some label'
)

I'm new to using the "splat operator" in PHP and I was wondering why this code is considered invalid:

$key = ...array_map( function( $el ) {
    return array(
        $el['value'] => $el['label']
    );
}, array(
    array(
        'label' => 'Some label',
        'value' => 'some-value'
    )
) );

But this code works fine, achieves my goal, but the array_merge is completely pointless:

$key = array_merge( array(), ...array_map( function( $el ) {
    return array(
        $el['value'] => $el['label']
    );
}, array(
    array(
        'label' => 'Some label',
        'value' => 'some-value'
    )
) ) );

Wrapping it in parentheses doesn't seem to help. Is this not what the splat operator was at least partially intended to do? It seems like a shame to have to pass it through a superfluous function to make it work.

Is there another PHP function I'm unaware of that does the same thing without the redundancy?

Answer 1

based ont this: https://wiki.php.net/rfc/spread_operator_for_array the spread operator is coming in 7.4.

we only have argument unpacking: https://wiki.php.net/rfc/argument_unpacking