Why can't I set zero as the first key in the array?

Question

The result of this code:

for($i = 0; $i <= 7; $i++){

    $eachone[] = array ('a' => '1', 'b' => '2', 'c' => '3');

$a[] = array($i => $eachone);

unset($eachone);

}

$json_string = json_encode($a);
echo $json_string;

is:

[
    [
        [
            {
                "a": "1",
                "b": "2",
                "c": "3"
            }
        ]
    ],
    {
        "1": [
            {
                "a": "1",
                "b": "2",
                "c": "3"
            }
        ]
    },
    {
        "2": [
            {
                "a": "1",
                "b": "2",
                "c": "3"
            }
        ]
    },
    {
        "3": [
            {
                "a": "1",
                "b": "2",
                "c": "3"
            }
        ]
    },
    {
        "4": [
            {
                "a": "1",
                "b": "2",
                "c": "3"
            }
        ]
    },
    {
        "5": [
            {
                "a": "1",
                "b": "2",
                "c": "3"
            }
        ]
    },
    {
        "6": [
            {
                "a": "1",
                "b": "2",
                "c": "3"
            }
        ]
    },
    {
        "7": [
            {
                "a": "1",
                "b": "2",
                "c": "3"
            }
        ]
    }
]

Can you notice how it's skipping the first number, which is zero? The question is: Why?