如果字符串包含字符串x位置中的字符，则追加并控制[递归]

Question

i have this situation:

$asd = "Hey hello world!";

now i would like to know how can i append (if not exist) recursively(every 3 chars) a char (ex: "J") ... parsing from left to right the string?

so results will be:

$asd = "HeyJ heJlloJ woJrldJ!";

Answer 1

How about:

$str = "Hey hello world!";
for($i=3; $i<strlen($str); $i+=4) {
    if ($str[$i] != 'J')
        $str = substr_replace($str, 'J', $i, 0);
}
echo $str,"
";

output:

HeyJ heJlloJ woJrldJ!