- substr: documentation ( source)
- strtoupper: documentation ( source)
<?php
echo uppercase_letters('ac84894987984987AD', 0, 2);
echo uppercase_letters('acaDdf984987Ad',0,10);
function uppercase_letters($str, $start, $end){
$strlen = strlen($str);
$str_uc = strtoupper(substr($str, $start, $end));
$then = substr($str, $end, $strlen);
return $str_uc . $then;
}