for($i=0;$i < length($string)/$len; $i++) {
    $codons[$i] = substr($string,$i*$len,$len);
}

##</code><code>##

$start=0;
for ($pos=0; $pos < length($string)-1;$pos++) {
    if ( substr($string,$pos,1) ne 
         substr($string,$pos+1,1) ) {
        $runs[$i++] = substr($string, $start,$pos-$start+1);
        $start=pos;
     } # end check for end of run
} # end loop across letters of string

$runs[$i++] = substr($string,$start,$pos-$start+1);