codice:
<?php
$str="ABCDEFGHI";
function strtolowerindex($str, $array) {
    $strret = "";
    if (($len = strlen($str)) != 0)
        for ($pos = 0; $pos < $len; $pos++)
            if (array_search($pos, $array)===false)
                $strret.=substr($str, $pos, 1);
            else
                $strret.=strtolower(substr($str, $pos, 1));
                
    return $strret;
}
echo strtolowerindex($str, array(4,5))."

";

//oppure

$str="ABCDE";
function strtolowerindexed($str, $array) {
    $len=strlen($str);
    sort($array);
    foreach($array as $value)
        if($len>$value)
            $str[$value]=strtolower ($str[$value]);
        else
            break;
    return $str;
}
echo strtolowerindexed($str, array(4,5))."

";

//oppure

$str="ABCDEFGHI";
$str[4]=strtolower($str[4]);
$str[5]=strtolower($str[5]);
echo $str;
?>