Теория: Распаковка аргументов

<?php

$arrayOfArrays = [
    [1, 2],
    [2, 3]
];

array_merge(...$arrayOfArrays);
# Array
# (
#     [0] => 1
#     [1] => 2
#     [2] => 2
#     [3] => 3
# )

<?php

array_merge($arrayOfArrays[0], $arrayOfArrays[1]);
// array_merge([1, 2], [2, 3]);

<?php

array_merge([3], ...$arrayOfArrays);
# Array
# (
#     [0] => 3
#     [1] => 1
#     [2] => 2
#     [3] => 2
#     [4] => 3
# )

<?php

$array = [3, 2];
array_merge([3], $array, ...$arrayOfArrays);
# Array
# (
#     [0] => 3
#     [1] => 3
#     [2] => 2
#     [3] => 1
#     [4] => 2
#     [5] => 2
#     [6] => 3
# )

<?php

array_merge(...$arrayOfArrays, [3, 2]);
// Fatal error: Cannot use positional argument after argument unpacking