PHP 配列の最大ネストの深さはどのように決定しますか?

$array = [
    'level1' => 'value1',
    'level2' => [
        'level3' => [
            'level4' => 'value4',
        ],
    ],
];

function array_depth($array) {
    $array_str = print_r($array, true);
    $lines = explode("\n", $array_str);

    $max_indentation = 1;

    foreach ($lines as $line) {
        $indentation = (strlen($line) - strlen(ltrim($line))) / 4;
        $max_indentation = max($max_indentation, $indentation);
    }

    return ceil(($max_indentation - 1) / 2) + 1;
}

echo array_depth($array); // Output: 4