not sure i understand the question. you want to search a string for another string and if found return the next character?
$string = '123456789';
$strpos = strpos($string, '5');
if ($strpos !== false)
{
if ($strpos == (strlen($string) - 1)) {echo 'match found at last character';}
else {echo $string{$strpos + 1};}
}
else
{
echo 'no match found';
}
this will output "6" because it follows "5"