PHP - syntax for list of arrays in a single variable

New Answer:

Based on the comments below I think I understand the problem. I can think of two solutions to this.

First solution: The first one is the easy one, but you need PHP 7.4+ for it to work.

You can use a new feature called the slant operator (...) where by you can "unpack" one array into another. Here is the example:

<?php
$sub_arrays = [array("middle" => "array"), array(), array("external"=>"array")];

$array = array(
    'template' => array(
        array(
            array(),
            array(
               array('first'=>"array"),
                ...$sub_arrays, // use it here
                array('last'=>"array")
            )
        ),
    ),
);

print_r($array);

Live demo: https://3v4l.org/s4Q8u

Second solution: This is a bit more complex that the first one. It involves wrapping the array into a function and dynamically creating the parent array of $sub_arrays. See this:

<?php
$sub_arrays = [array("middle" => "array"), array(), array("external"=>"array")];

function create_array($sub) {
    $parent_array[] = array('first'=>"array");

    foreach($sub as $s)
    {
         $parent_array[] = $s;
    }

    $parent_array[] = array('last'=>"array");

    $array = array(
        'template' => array(
            array(
                array(),
                $parent_array
            ),
        ),
    );

    return $array;
}
print_r(create_array($sub_arrays));

Live demo: https://3v4l.org/1Vahd

I hope this helps.

Old Answer:

Your questions leaves out a lot of details to understand why you are trying to do this.

For example:

Wrapping the list of arrays in an array while declaring the variable isn't an option,

how is $sub_array inserted in $array, at what stage, are they inside a class or a function?

In any case, from what I understood, I think list is what you are looking for.

See this example:

<?php
list($a, $b) = [array('example' => 'test'), array('another' => 'array')];


var_dump($a, $b);

you can also use list for nested arrays like so:

list($a, list($b, $c)) = [array('example' => 'test'), array(array('another' => 'array'), array('I am nested'=>'array'))];

var_dump($a, $b, $c);

Im not sure what you are asking for, but if it is how to create array variables and use them later, then here is an example. I took the WordPress example you linked in the comments. This is the template arrays broken down into sections. The structure seems to follow a pattern, hopefully this clarifies it.

$templateTop = array(
    'core/paragraph'        =>  array(
        'placeholder'           =>  'Add a root-level paragraph',
    )
);

$column1 = array(
    'core/column',
    array(),
    array(
        array(
            'core/image',
            array()
        ),
    )
);

$column2 = array(
    'core/column',
    array(),
    array(
        array(
            'core/paragraph',
            array(
                'placeholder' => 'Add a inner paragraph'
            )
        ),
    )
);

$templateColumns = array(
    'core/columns',
    array(),
    array(
        $column1, $column2
    )
);

$template = array(
    $templateTop, $templateColumns
);

You may prefer the shorter array syntax available since PHP 5.4

<?php

$arr1 = ['a', 'b', 'c'];      // >= 5.4
$arr2 = array('a', 'b', 'c'); //  < 5.4

See here for information about short syntax and arrays in general: https://www.php.net/manual/en/language.types.array.php

Using the short syntax your nested array may look like this:

<?php

$arr = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9],
];

The example above is a two dimensional nested array. The outer array has three elements which are arrays itself.

In order to include / require an array (or any other value) from another PHP file just do it like so $returnValue = require('./other_file.php').

In other_file.php something like this will work:

<?php

return [
    [
        [1, 2, 3],
        [4, 5, 6],
    ],
    [
        [9, 8, 7],
        [6, 5, 4],
    ],
];

Hope this helps.

I'm going to attempt to simplify the problem, and define some variables as follows

$firstArray = $a; 
$subArrays = [$i, $j, $k]; 
$lastArray = $z;

where $a, $i, $j, $k, and $z are all arrays

I believe what you are trying to achieve is

$result = [$a, $i, $j, $k, $z]

without referencing the variables $a, $i, $j, $k, or $z

This can be achieved using array_merge(), providing that the arrays are not associative with duplicate keys

$result = array_merge([$firstArray], $subArrays, [$lastArray]);

Which is equivalent to

$result = array_merge([$a], [$i, $j, $k], [$z]);

By wrapping the $firstArray and $lastArray in arrays of their own, this puts them at the same level of nesting as the $subArrays before combining them all into one array

Applying this all to your original problem, and using the longer syntax, results in

$sub_arrays = array(array(), array(), array()); // Wrapped

$array = array(
    'template' => array(
        array(
            array(),
            array_merge(
                array(array()), // Wrapped
                $sub_arrays,
                array(array()), // Wrapped
            ),
        ),
    ),
);