php字符串word末字符大小写互换，字符串大小写要求给出一个字符串如 “a journey of, a thousand 'miles' must can't \begin\ with a single step.” ，通过 php 程序处理变成 “a journey of, a thousand 'miles' must can't begin with a single step.”
注意：
1、每个单词最后的字符如果是大写就变成小写，如果是小写就变成大写。
2、需要考虑类似  can't 这种形式的转换。
3、标点符号（只考虑 , ' . ;）不用变化。
参考算法 测试
结果：
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 bresult:a journey of, a thousand 'miles' must can't begin with a single step. a bsource: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 bresult:a journey of, a thousand 'miles' must can't begin with a single step. a b

我们可以看到，是符合预期的。
题目来源http://blog.sijiaomao.com/?p=98，有改动（增加了can't这种），按改后的规则，原文答案全是错的。
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