Task: Huffman coding
C++: #include <iostream>
#include <queue>
#include <map>
#include <climits> 
#include <iterator>
#include <algorithm>

const int UniqueSymbols = 1 << CHAR_BIT;
const char* SampleString = "this is an example for huffman encoding";

typedef std::vector<bool> HuffCode;
typedef std::map<char, HuffCode> HuffCodeMap;

class INode
{
public:
    const int f;

    virtual ~INode() {}

protected:
    INode(int f) : f(f) {}
};

class InternalNode : public INode
{
public:
    INode *const left;
    INode *const right;

    InternalNode(INode* c0, INode* c1) : INode(c0->f + c1->f), left(c0), right(c1) {}
    ~InternalNode()
    {
        delete left;
        delete right;
    }
};

class LeafNode : public INode
{
public:
    const char c;

    LeafNode(int f, char c) : INode(f), c(c) {}
};

struct NodeCmp
{
    bool operator()(const INode* lhs, const INode* rhs) const { return lhs->f > rhs->f; }
};

INode* BuildTree(const int (&frequencies)[UniqueSymbols])
{
    std::priority_queue<INode*, std::vector<INode*>, NodeCmp> trees;

    for (int i = 0; i < UniqueSymbols; ++i)
    {
        if(frequencies[i] != 0)
            trees.push(new LeafNode(frequencies[i], (char)i));
    }
    while (trees.size() > 1)
    {
        INode* childR = trees.top();
        trees.pop();

        INode* childL = trees.top();
        trees.pop();

        INode* parent = new InternalNode(childR, childL);
        trees.push(parent);
    }
    return trees.top();
}

void GenerateCodes(const INode* node, const HuffCode& prefix, HuffCodeMap& outCodes)
{
    if (const LeafNode* lf = dynamic_cast<const LeafNode*>(node))
    {
        outCodes[lf->c] = prefix;
    }
    else if (const InternalNode* in = dynamic_cast<const InternalNode*>(node))
    {
        HuffCode leftPrefix = prefix;
        leftPrefix.push_back(false);
        GenerateCodes(in->left, leftPrefix, outCodes);

        HuffCode rightPrefix = prefix;
        rightPrefix.push_back(true);
        GenerateCodes(in->right, rightPrefix, outCodes);
    }
}

int main()
{
    
    int frequencies[UniqueSymbols] = {0};
    const char* ptr = SampleString;
    while (*ptr != '\0')
        ++frequencies[*ptr++];

    INode* root = BuildTree(frequencies);
    
    HuffCodeMap codes;
    GenerateCodes(root, HuffCode(), codes);
    delete root;

    for (HuffCodeMap::const_iterator it = codes.begin(); it != codes.end(); ++it)
    {
        std::cout << it->first << " ";
        std::copy(it->second.begin(), it->second.end(),
                  std::ostream_iterator<bool>(std::cout));
        std::cout << std::endl;
    }
    return 0;
}

PHP: <?php
function encode($symb2freq) {
    $heap = new SplPriorityQueue;
    $heap->setExtractFlags(SplPriorityQueue::EXTR_BOTH);
    foreach ($symb2freq as $sym => $wt)
        $heap->insert(array($sym => ''), -$wt);

    while ($heap->count() > 1) {
        $lo = $heap->extract();
        $hi = $heap->extract();
        foreach ($lo['data'] as &$x)
            $x = '0'.$x;
        foreach ($hi['data'] as &$x)
            $x = '1'.$x;
        $heap->insert($lo['data'] + $hi['data'],
                      $lo['priority'] + $hi['priority']);
    }
    $result = $heap->extract();
    return $result['data'];
}

$txt = 'this is an example for huffman encoding';
$symb2freq = array_count_values(str_split($txt));
$huff = encode($symb2freq);
echo "Symbol\tWeight\tHuffman Code\n";
foreach ($huff as $sym => $code)
    echo "$sym\t$symb2freq[$sym]\t$code\n";
?>