PHP字符串word末字符实现大小写互换的方法

<?php
    function convertLastChar($str) {
        $markArr = array(", ", "' ", "\" ", ". ", "; ");
        $ret = "";
        for ($i = 0, $j = strlen($str); $i < $j; $i++) {
            if ($i < $j - 2) {
                $afterStr = $str{$i + 1} . $str{$i + 2};
            } else if ($i < $j - 1) {
                $afterStr = $str{$i + 1} . " ";
            }
            if (in_array($afterStr, $markArr)
                || $i == $j - 1
                || $str{$i + 1} == " ") {
                $ret .= strtoupper($str{$i}) === $str{$i}
                    ? strtolower($str{$i})
                    : strtoupper($str{$i});
            } else {
                $ret .= $str{$i};
            }
        }
        return $ret;
    }
?>

<?php

    //test
    $str1 = "A journey of, a thousand 'miles' must can't \"begin\" with a single step.";
    $str2 = "A journey of, a thousand 'miles' must can't \"begin\" with a single step. ";
    $str3 = "A journey of, a thousand 'miles' must can't \"begin\" with a single step. a ";
    $str4 = "A journey of, a thousand 'miles' must can't \"begin\" with a single step. a B";
    $str5 = "A journey of, a thousand 'miles' must can't \"begin\" with a single step. a b'";
    $str6 = "A journey of, a thousand 'miles' must can't \"begin\" with a single step. a B\"";

    echo "source:<br/>" . $str1 . "<br/>result:<br/>" . convertLastChar($str1) . "<br/><br/>";
    echo "source:<br/>" . $str2 . "<br/>result:<br/>" . convertLastChar($str2) . "<br/><br/>";
    echo "source:<br/>" . $str3 . "<br/>result:<br/>" . convertLastChar($str3) . "<br/><br/>";
    echo "source:<br/>" . $str4 . "<br/>result:<br/>" . convertLastChar($str4) . "<br/><br/>";
    echo "source:<br/>" . $str5 . "<br/>result:<br/>" . convertLastChar($str5) . "<br/><br/>";
    echo "source:<br/>" . $str6 . "<br/>result:<br/>" . convertLastChar($str6) . "<br/><br/>";
?>

source:
A journey of, a thousand 'miles' must can't "begin" with a single step.
result:
a journeY oF, A thousanD 'mileS' musT can'T "begiN" witH A singlE steP.

source:
A journey of, a thousand 'miles' must can't "begin" with a single step.
result:
a journeY oF, A thousanD 'mileS' musT can'T "begiN" witH A singlE steP.

source:
A journey of, a thousand 'miles' must can't "begin" with a single step. a
result:
a journeY oF, A thousanD 'mileS' musT can'T "begiN" witH A singlE steP. A

source:
A journey of, a thousand 'miles' must can't "begin" with a single step. a B
result:
a journeY oF, A thousanD 'mileS' musT can'T "begiN" witH A singlE steP. A b

source:
A journey of, a thousand 'miles' must can't "begin" with a single step. a b'
result:
a journeY oF, A thousanD 'mileS' musT can'T "begiN" witH A singlE steP. A B'

source:
A journey of, a thousand 'miles' must can't "begin" with a single step. a B"
result:
a journeY oF, A thousanD 'mileS' musT can'T "begiN" witH A singlE steP. A b"