<?php
// block 100,004
$txids = array(
    'AB7ED423933FE5413DC51B1041A58FD8AF0CD70491B1CE607CB41DDDCE74A92B',
    'C763E59A79C9F1E626DDF1C3E9F20F234959C457FF82918F7B24E9D18A12DB99',
    '80EA3E647AEEF92973F5414E0CAA1721C7B42345B99ED161DD505ACC41F5516B',
    '1B72EEFD70CE0A362EC0CB48E2213274DF3C55F9DABD5806CDC087A335498CD3',
    '23E13370F6D59E2D7C7A9CA604B872312DE34A387BD7DECA2C8F4486F7E66173',
    '149A098D6261B7F9359A572D797C4A41B62378836A14093912618B15644BA402',
);


// ------------
// PREPARE DATA
// ------------

// Convert txids in to big endian (BE), because that's the format they need to be hashed in to get the merkle root.
foreach ($txids as $txid) {
    $txidsBE[] = implode('', array_reverse(str_split($txid, 2)));
}

// Now convert each of these txids in to binary, because the hash function wants the binary value, not the hex.
foreach ($txidsBE as $txidBE) {
    $txidsBEbinary[] = hex2bin($txidBE);
}


// -----------------------------
// RECURSIVE FUNCTION STARTS NOW
// -----------------------------
// Now the TXIDs are ready. So run through the array, concatenating each pair and hashing them.

function merkleroot($txids) {

    while (count($txids) > 0) {

        if (count($txids) >= 2) {
            // Get first two
            $pair_first = $txids[0];
            $pair_second = $txids[1];

            // Hash them
            $pair = $pair_first.$pair_second;
            $pairhashes[] = hash('sha256', $pair);

            // Remove those two from the array
            unset($txids[0]);
            unset($txids[1]);
            // Re-set the indexes (the above just nullifies the values) and make a new array without the original first two slots.
            $txids = array_values($txids);
        }

        if (count($txids) == 1) {
        // Get the first one twice
            $pair_first = $txids[0];
            $pair_second = $txids[0];

            // Hash it with itself
            $pair = $pair_first.$pair_second;
            $pairhashes[] = hash('sha256', $pair);

            // Remove it from the array
            unset($txids[0]);
            // Re-set the indexes (the above just nullifies the values) and make a new array without the original first two slots.
            $txids = array_values($txids);
        }

    }

    return $pairhashes;

}


// Test
print_r($txids);
print_r(merkleroot($txidsBE));