php - PHP中的爆炸字符串用于将一部分转换为大写，另一部分转换为小写？

Question

有谁知道如何在 PHP 中分解一些字符串，例如：

<?
// Submitted through register form

$first_name = $_POST['first_name'];

// example $_POST['first_name'] is somebody
// and need to be exploded at position 1 or by letter 'o' in this example
// but that is not always letter o
// I made also 

$first_name_lc = strtolower($first_name);

// converted all to lowers and now
// exploding at letter o

$explode_rule = 'o';

$first_name_lc_exploded = explode($explode_rule, $first_name_lc);

// now in example we have $first_name_lc_exploded[0] and $first_name_lc_exploded[1]
// $first_name_lc_exploded[0] is now 's' and $first_name_lc_exploded[1] is 'mebody'
// now new first name is $first_name_lc_exploded[0].$explode_rule.$first_name_lc_exploded[1]
// but $first_name_lc_exploded[0] need to be converted in upper and only that, nothing else
// than $first_name_lc_exploded[0] is 'S' instead of 's'
// and first_name is now 'Somebody' instead of 'somebody'
?>

所以现在，问题是：如何用在字符串中查找第二个字母的东西替换字母 o，并用那个规则，一些变量来分解它。有人有想法吗？