split string after x characters

If you need to split a string after every 5 characters, try str_split():

$output = str_split($string, 5);

If you only need to extract the first 5 characters and put the rest of the string in the second part of your array, you can use substr() as NullUserException suggests (code from his now-deleted answer):

$output[0] = substr($string, 0, 5);
$output[1] = substr($string, 5);