Multi dimensional associative array in php

I need some guide to associative array below, how do i access or retrieve specified value,for example the height under the student_name "Kevin" in the student_details in JSON format.

<?php
$college=[
    'college_id' => '123',
    'student_details'=> [
        'student_name'=>[
            'Kevin'=> [
                'height'=>'170',
                'weight'=>'65',
            ],
            'Daniel'=> [
                'height'=>'170',
                'weight'=>'65',
            ] ,
            'Paul'=> [
                'height'=>'179',
                'weight'=>'70',
            ]
        ]
    ]
];
echo json_encode($college);
?>