<?php

$arr = array();

$arr[1]["9:00"]["10a"]="Matemātika";

print_r($arr);

var_dump(isset($arr[1]["9:00"]["10a"]));
var_dump(isset($arr[1]["9:00"]["9a"]));

Array
(
    [1] => Array
        (
            [9:00] => Array
                (
                    [10a] => Matemātika
                )

        )

)
bool(true)
bool(false)