parseTOC.php

<?php

function parseTOC($items) {
    if (count($items) == 0) return [];

    reset($items);
    $stack = array();
    $ret   = array();

    $stack[] = &$ret;
    $first   = array(
        'level' => current($items)[0],
        'text'  => current($items)[1],
        'subItems' => []
    );
    $ret[] = &$first;
    $stack[] = &$first;
    next($items);

    while (current($items)) {
        $cur = array(
            'level' => current($items)[0],
            'text'  => current($items)[1],
            'subItems' => []
        );

        if ($cur['level'] < $stack[count($stack)-1]['level']) {
            while ($stack[count($stack)-1] != $ret && $cur['level'] < $stack[count($stack)-1]['level']) {
                array_pop($stack);
            }
            continue;
        }

        if ($cur['level'] == $stack[count($stack)-1]['level']) {
            array_pop($stack);
            if ($stack[count($stack)-1] == $ret) {
                $ret[] = $cur;
                $stack[] = &$ret[count($ret)-1];
            } else {
                $stack[count($stack)-1]['subItems'][] = $cur;
                $stack[] = &$stack[count($stack)-1]['subItems'][ count($stack[count($stack)-1]['subItems'])-1 ];
            }
            next($items);
            continue;
        }

        if ($cur['level'] > $stack[count($stack)-1]['level']) {
            $stack[count($stack)-1]['subItems'][] = $cur;
            $stack[] = &$stack[count($stack)-1]['subItems'][ count($stack[count($stack)-1]['subItems'])-1 ];
            next($items);
        }
    }
    return $ret;
}

echo json_encode(parseTOC(array(
    array(1, 'textA'),
    array(2, 'text'),
    array(3, 'text'),
    array(5, 'text'),
    array(1, 'textB'),
    array(3, 'text'),
    array(2, 'text'),
    array(2, 'text'),
    array(3, 'text'),
)), JSON_PRETTY_PRINT);

//echo json_encode(parseTOC(array(
//    array(1, 'textB'),
//    array(3, 'text'),
//    array(2, 'text'),
//    array(2, 'text'),
//    array(3, 'text'),
//)), JSON_PRETTY_PRINT);

/* output:
[
    {
        "level": 1,
        "text": "textA",
        "subItems": [
            {
                "level": 2,
                "text": "text",
                "subItems": [
                    {
                        "level": 3,
                        "text": "text",
                        "subItems": [
                            {
                                "level": 5,
                                "text": "text",
                                "subItems": []
                            }
                        ]
                    }
                ]
            }
        ]
    },
    {
        "level": 1,
        "text": "textB",
        "subItems": [
            {
                "level": 3,
                "text": "text",
                "subItems": []
            },
            {
                "level": 2,
                "text": "text",
                "subItems": []
            },
            {
                "level": 2,
                "text": "text",
                "subItems": [
                    {
                        "level": 3,
                        "text": "text",
                        "subItems": []
                    }
                ]
            }
        ]
    }
]
*/