function convert_part_to_uppercase($s, $idx) {
$len = strlen($s);
if ($idx < 0 || $idx > $len) {
return s;
}
for ($i = 0; $i < $len; $i++) {
if ($i >= $idx && ($s[$i] >= 'a' && $s[$i] <= 'z')) {
$s[$i] = strtoupper($s[$i]);
}
}
return $s;
}
$s = "php programming";
$s = convert_part_to_uppercase($s, 5);
echo $s;
/*
run:
php pROGRAMMING
*/