将数组转化为json格式PHP

$student = array(
    'name' => 'Sam',
    'age' => 18,
    'school' => 'ABC High School'
);
$json_str = json_encode($student);
echo $json_str;

{
    "name":"Sam",
    "age":18,
    "school":"ABC High School"
}

$class = array(
    array('name' => 'John', 'age' => 17, 'school' => 'ABC High School'),
    array('name' => 'Mike', 'age' => 18, 'school' => 'DEF High School'),
    array('name' => 'Lucy', 'age' => 16, 'school' => 'GHI High School')
);

$json_str = json_encode($class, JSON_FORCE_OBJECT | JSON_PRETTY_PRINT);
echo $json_str;

{
    "0": {
        "name": "John",
        "age": 17,
        "school": "ABC High School"
    },
    "1": {
        "name": "Mike",
        "age": 18,
        "school": "DEF High School"
    },
    "2": {
        "name": "Lucy",
        "age": 16,
        "school": "GHI High School"
    }
}