It's possible to define keys only for some of the array items

$array = [1, 2, "name"=>"Three", 4];
print_r($array);

Result

Array
(
    [0] => 1
    [1] => 2
    [name] => Three
    [2] => 4
)

If we set higher index manually, it will continue adding 1 to the max index

$array = [1, 2, 7=>"Three", 4];
print_r($array);

Result

Array
(
    [0] => 1
    [1] => 2
    [7] => Three
    [8] => 4
)

If we set higher index manually, it will continue adding 1 to the index

$array = [1, 2, 7=>"Three", 4=>"forth", 25];
print_r($array);

Result

Array
(
    [0] => 1
    [1] => 2
    [7] => Three
    [4] => forth
    [8] => 25
)

Only integer and string values are allowed as array keys

$array = [
	1    => "a",
    "1"  => "b",
    "-1"  => "-b",
    '1.5'  => "c",
    true => "d",
];
print_r($array);

Result

Array
(
    [1] => d
    [-1] => -b
    [1.5] => c
)