<?php
// AndrewMohawk
// [email protected]
// http://www.andrewmohawk.com

/* Decode Track 2/3 data from binary */
$binary = "<yourBinaryHere>";

// this function by mtroy dot student at gmail dot com taken from http://php.net/manual/en/function.strpos.php
function strpos_r($haystack, $needle)
{
    if(strlen($needle) > strlen($haystack))
        trigger_error(sprintf("%s: length of argument 2 must be <= argument 1", __FUNCTION__), E_USER_WARNING);

    $seeks = array();
    while($seek = strrpos($haystack, $needle))
    {
        array_push($seeks, $seek);
        $haystack = substr($haystack, 0, $seek);
    }
    return $seeks;
}

function processBinary($binary)
{
    $AsciiOutput = "";

    //find start sentinal
    $start_sentinal = strpos($binary,"11010");
    if($start_sentinal === false)
    {
        echo "Could not find start sentinal\n";
        return false;
    }

    //find end sentinal
    $end_sentinal = false;
    $end_sentinals = strpos_r($binary,"11111");
    if(count($end_sentinals) == 0)
    {
        echo "Could not find end sentinal\n";
        return false;
    }

    //Check end sentinal is on a 5 bit boundry
    foreach($end_sentinals as $es)
    {
        $es = $es;
        if(($es - $start_sentinal) % 5 == 0)
        {
            $end_sentinal = $es;
        }
    }

    if($end_sentinal == false)
    {
        echo "End sentinal not on correct boundry\n";
        return false;
    }



    //Lets decode the data:
    $bit_length = 5; // 4 bits for data, 1 bit for odd-parity or LRC checking


    $data = substr($binary,$start_sentinal,($end_sentinal-$start_sentinal+5));

    $currentBits = "";
    $currentNum = 0;
    $finalString = "";

    for($i=0;$i<strlen($data);$i++)
    {
        if(strlen($currentBits) < $bit_length)
        {
            $currentBits .= $data[$i];

        }

        if(strlen($currentBits) == $bit_length)
        {
            $parityBit = $currentBits[4];
            $dataBits = substr($currentBits,0,4);

            $asciiChar = 0;


            for($x=0;$x<4;$x++)
            {
                $currentNum += $dataBits[$x];
            }



            $dec = bindec($dataBits);
            $dec = str_pad($dec, 2, "0", STR_PAD_LEFT); // just so output is nice
            $asciiChar = chr(bindec(strrev($dataBits))+48); // reverse the binary (since its LSB first) then convert to dec, add 48 and then take it to ASCII
            echo "$currentBits - Data ($dataBits) Parity($parityBit) Decimal ($dec) Ascii($asciiChar)";
            if(($currentNum + $parityBit) % 2 == false)
            {
                echo " __ Parity: Invalid";
            }
            else
            {
                echo " __ Parity: Valid";
            }
            $AsciiOutput .= $asciiChar;
            echo "\n";
            $currentBits = "";
            $currentNum = 0;

        }


    }
    echo "\n\nTotal Out (ascii): $AsciiOutput\n";
}
echo "Trying One way:\n\n";
if (processBinary($binary) == false)
{
    //reverse.
    echo "\n\n";
    echo "Trying The Reverse:\n\n";
    processBinary(strrev($binary));
}