History grounds us in our roots. History is an important and interesting field of study, and learning the history of our home country can give us a deeper, more meaningful glimpse into our ancestral pasts, and how we got to where we are today. Many people feel like they need a sense of cultural belonging, which is something that studying your roots and being open-minded to the evolution of your culture can provide.
trim：去除字符串的前后空格 ltrim:left rtrim right
‘abc’= trim(‘ abc ‘);
rtrim();
trim();
upper:
lower:
Ucfirst:
查找字符串首次出现位置：strpos string pos
截取：
替换
字符串转数字
substr
Substr(str,start,len)
周国平
如果start大于总字符，返回空。
如果len大于了剩余的字符数或如果len为空，将len重置为：
2，6
如果我想取最后一个字，-2,
如果不存在参数len，那么返回自start位置开始至最后的所有字符，等同于将len设为总字符数减去start,然后再加1.
如果start是负数，将start重置为正数 str的总字符数减去start的绝对值。
必须对start及len确定好值。
将结果变量设为空，
设循环变量i设为start
将结果设为连接结果和str的第i个字符
将i增加1，重复len次
Substr(‘周国平’,2,1)
伪代码
