explode but by counting characters?

[viveleroi0]

How can I split a ten character string into three parts by counting the characters?

I want to seperate the first three, second three, and last four into an array.

Like a telephone number.

How do I do that?

[Installer]

$num[] = substr($str, 0, 3);
$num[] = substr($str, 3, 3);
$num[] = substr($str, -4);