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

function compress( $str) {
$strArr = str_split($str.'0');
$count = 0;
$resStr = '';
$strCheck = $strArr[0];
foreach($strArr as $key => $value)
{
    if($strCheck == $value)
    {
       $count++;
    } 
    else
    {
        if($count == 1)
        {
            $strCheck = $value;
            $resStr .= $strArr[$key-1];
            $count=1;
        }
        elseif($count == 2)
        {
            $strCheck = $value;
            $resStr .= $strArr[$key-1].$strArr[$key-1];
            $count=1;
        }
        else
        {
            $strCheck = $value;
            $resStr .= $strArr[$key-1].$count;
            $count=1;
        }
    } 

} 
return $resStr;

}