Simple Markov Chain in PHP

Here’s a simple Markov chain implementation in PHP, loosely adapted from this excellent write up about implementing Markov chains in javascript:

class Link {
    private $nexts = array();
    public function addNextWord($word) {
        if (!is_string($word)) {
            throw new Exception('addNextWord method in Link class is run with an string parameter');
        }
        if (!isset($this->nexts[$word])) {
            $this->nexts[$word] = 0;
        }
        $this->nexts[$word]++;
    }
    public function getNextWord() {
        $total = 0;
        foreach($this->nexts as $word => $count) {
            $total += $count;
        }
        $randomIndex = rand(1, $total);
        $total = 0;
        foreach($this->nexts as $word => $count) {
            $total += $count;
            if ($total >= $randomIndex) {
                return $word;
            }
        }
    }
}

class Chain {
    private $words = array();
    function __construct($words) {
        if (!is_array($words)) {
            throw new Exception('Chain class is instantiated with an array');
        }

        for($i = 0; $i < count($words); $i++) {
            $word = (string) $words[$i];
            if (!isset($this->words[$word])) {
                $this->words[$word] = new Link();
            }
            if (isset($words[$i + 1])) {
                $this->words[$word]->addNextWord($words[$i + 1]);
            }
        }
    }
    public function getChainOfLength($word, $i) {
        if (!is_string($word)) {
            throw new Exception('getChainOfLength method in Chain class is run with an string parameter');
        }
        if (!is_integer($i)) {
            throw new Exception('getChainOfLength method should be called with an integer');
        }
        if (!isset($this->words[$word])) {
            return '';
        } else {
            $chain = array($word);
            for ($j = 0; $j < $i; $j++) {
                $word = $this->words[$word]->getNextWord();
                $chain[] = $word;
            }
            return implode(' ', $chain);
        }
    }
}

And here is an example of usage:

function get_all_words_in_file($file) {
    return preg_split('/s+/ ', file_get_contents($file));
}

$file = 'testtext2.txt';

$words = get_all_words_in_file($file);
$chain = new Chain($words);
$newSentence = $chain->getChainOfLength('The', 200);
echo wordwrap($newSentence, 80, "n");

Conceptually, a Markov chain captures the idea of likelihood of traversing from state to state. You can populate this data for a block of text by passing through a block of text and counting the number of occurrences of words that follow a given word. You can then use this data to generate new blocks of text.