Strings are array chars, so you can get each char of them by iterating through their length
define('SERIAL_NUM_LEN', 4);
$info1 = 'nkw549blc003i00021';
$number1 = ''; $number2 = '';
for ($i = 0; $i < strlen($info1)-SERIAL_NUM_LEN; $i++) {
$number1 .= $info1[$i];
}
for ($i = strlen($info1)-SERIAL_NUM_LEN; $i < strlen($info1); $i++) {
$number2 .= $info1[$i];
}
var_dump($number1, $number2);
Output:
string 'nkw549blc003i0' (length=14)
string '0021' (length=4)
This way you can skip whichever chars from the string you want if you want to build totally different string. Or add chars in the middle.