dummydb.php

<?php

function build_index($source_file = 'source.txt', $index_file = 'index.dat')
{
    $offset = 0;
    $lenght = 0;

    $index_fh = fopen($index_file, 'w');
    $source_fh = fopen($source_file, 'r');

    while (($buff = fgets($source_fh)) !== false)
    {
        $length = strlen($buff);
        // pack data and write to index.
        // L unsigned long (always 32 bit, machine byte order)
        // http://www.php.net/manual/en/function.pack.php
        fwrite($index_fh, pack('LL', $offset, $length));
        $offset += $length;
    }
    fclose($source_fh);
    fclose($index_fh);
}

// Размер придётся захардкодить или добавить подсчёт
function get_data($source_file = 'source.txt', $index_file = 'index.dat', $string_count)
{
    // 2 * 4 byte
    $index_data_size = 8;
    $n = rand(0, $string_count - 1);

    // get offset:lenght pair from index
    $index_fh = fopen($index_file, 'r');
    fseek($index_fh, $n * $index_data_size);
    $raw = unpack('L2', fread($index_fh, $index_data_size));
    fclose($index_fh);

    // print_r($raw);
    $offset = $raw[1];
    $lenght = $raw[2];

    // get string from source
    $source_fh = fopen($source_file, 'r');
    fseek($source_fh, $offset);
    $data = trim(fread($source_fh, $lenght));
    fclose($source_fh);

    return $data;
}