php实现word末尾字符大小写互换代码

<?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/>";
?>