php array push adding that data in other code block

i want to array push some data to my main array. but it add my push data incorrectly. i have below code.

$customKeyboard = array();
$customKeyboard = [
    "inline_keyboard" => [
        [["text" => "back 🔙", "callback_data" => "back"]]
    ]
];

then I do this

$customKeyboard[] = [["text" => "test", "callback_data" => "test"]];

and i want to $customKeyboard[] data will result as below

$customKeyboard = [
    "inline_keyboard" => [
        [["text" => "back 🔙", "callback_data" => "back"]],
        [["text" => "test", "callback_data" => "test"]]
    ]
];

You have to properly tell the system under what level / hierarchy you want to add the new data

So , change the line

$customKeyboard[] = [["text" => "test", "callback_data" => "test"]];

to

$customKeyboard["inline_keyboard"][] = [["text" => "test", "callback_data" => "test"]];

So, use the following

<?php
$customKeyboard = array();
$customKeyboard = [
    "inline_keyboard" => [
        [["text" => "back 🔙", "callback_data" => "back"]]
    ]
];

$customKeyboard["inline_keyboard"][] = [["text" => "test", "callback_data" => "test"]];
?>

See Demo