how to add sub array to main array in php laravel

Do this:

$json = array(
 'att1' => '1',
 'att2' => '2',
 'details' => null
);

Arrange your details array here.

$detailsArray =  [
    [
        "a" => 'value',
        "b" => 'value'
    ],
    [
        "a" => 'value',
        "b" => 'value'
    ]
];

Just put your data into your required array field

$json['details'] = $detailsArray;

You'll need to use PHP built-in function json_encode for converting it into JSON.

$json = json_encode($json); 

print_r($json);

The output is exactly what you need

{
"att1" : "1",
"att2" : "2",
"detail":[
    {
        "a": "value"
        "b": "value"
    },
    {
        "a": "value"
        "b": "value"
    }
  ]
}

when you have a json , you must decode it to get your object:

$object = json_decode($json);

then: you can work with decoded object as normal object:

$object->details = $detailsArray;

https://stackoverflow.com/a/55151110/10573560

short tutorial in:

https://www.w3schools.com/php/func_json_decode.asp

foreach ($someArr as $arrElement) {
  $json['detail'][] = array('a' => $arrElement, 'b' => $arrElement);
}

$result = json_encode($json);

You question is not really clear. Is this what you're looking for?