How to insert a new key value pair in array in php?

To add:

$arr["key"] = "value";

Then simply return $arr

Can't return directly like this way return $arr["key"] = "value";

foreach($test_package_data as $key=>$data ) {

   $category_detail_arr = $test_package_data[$key]['category_detail'];

   foreach( $category_detail_arr as $i=>$value ) {
     $test_package_data[$key]['category_detail'][$i]['count'] = $some_value;////<----Here
   }

}

If you are creating new array then try this :

$arr = ['key' => 'value'];

And if array is already created then try this :

$arr['key'] = 'value';