Insert a character after a first character in PHP

I want to insert a character after the first character in a word using PHP like this, ID: R-CDA12345 to first character in this ID serves as the type of an item so I can easily know where it is from.

I tried a method like this,

$id_number = "RCDA12345";

$char = explode(' ', $id_number);
$char[1] = "-";
var_export($char);

It’s not working -_-

>Solution :

You can do that with substr:

$char = $id_number[0] . '-' . substr($id_number, 1);

Or with substr_replace:

$char = substr_replace($id_number, '-', 1, 0);

Which can also insert string in string by given offset and with given length (3rd and 4th parameters)