Encode/compress sequence of repeating integers

Question

I have very long integer sequences that look like this (arbitrary length!):

0000000001110002220033333

Now I need some algorithm to convert

Answer 1

$str="0000000001110002220033333";

//$c will count the number of occurances.

$c=1;

$lastInt=substr($str,0,1);

$str=substr($str,1);

$resultStr='';

$loopEnd=strlen($str);


for($i=1; $i<=$loopEnd+1;$i++)

{

    $nowInt=substr($str,0,1);   
    if($lastInt==$nowInt)
    {
        $c++;
        $str=substr($str,1);
    }
    else
    {
        $char=chr((int)$lastInt + 97);
        $resultStr=$resultStr.$char.$c;
        $str=substr($str,1);
        $c=1;
        $lastInt=$nowInt;
    }
}

// we use if condition since for loop will not take the last integer if it repeats.

if($c>1)
{

$char=chr((int)$lastInt + 97);

$resultStr=$resultStr.$char.$c;

}

echo $resultStr;