For PHP 7.1 on, the documentation states that integer and string keys can be mixed, but that elements with and without keys cannot. Here is an example, using data from getimagesize() with mixed keys:

<?php
    $data=[
        0=> 160,
        1 => 120,
        2 => 2,
        3 => 'width="160" height="120"',
        'mime' => 'image/jpeg'
    ];
    list(0=>$width,1=>$height,2=>$type,3=>$dimensions,'mime'=>$mime)=$data;
?>

Here, the numeric keys also need to be specified, as if the whole array is treated as an associative array.

As noted elsewhere, the list() operator can be written in array format:

<?php
    [0=>$width,1=>$height,2=>$type,3=>$dimensions,'mime'=>$mime]=$data;
?>