3.字符串比较
integer strcmp(sting str1,string str2) //str1大于str2返回-1 str1小于str2返回1 str1和str2相等返回0
integer strmcmp(sting str1,string str2,integer length) //第三个参数限制length个字符的比较 

print strcmp("aardvark","aardwolf"); 
print strncmp("aardvark","aardwolf",4); 
---------------------输出结果---------------------------------------- 
-10 

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strcasecmp()和strncasecmp()是不区分大小写的比较函数

4.查找和抽取子字符串

string substr(sting source,integer start[,integer length])//从start开始取length个字符
start和length可以使用负值 

$var = "abcdefgh"; 
print substr($var,2);//从0开始计数 
print substr($var,2,3); 
print substr($var,-1);//从字符串的末尾开始 
print substr($var,-5,2); 
print substr($var,-5,-2); 
---------------------输出结果---------------------------------------- 
cdefgh 
cde 
h 
de 
def 

---------------------------------------------------------------------

integer strpos(string haystack,string needle[,integer offset])//查找子字符串的位置，返回第一次出现.
integer strrpos(string haystack,string needle)//只搜索单个字符（多个字符只取第一个），返回最后一次出现的索引.
还有常见的从 字符串中抽取找到的部分 的函数
string strstr(string haystack,string needle)//不区分大小写
string stristr(string haystack,string needle)//区分大小写
string strrchr(string haystack,sting needle)
*********** array explode(string separator,string subject[,integer limit])//返回一个字符串数组
array implode(string glue,array pieces)//返回一个字符串
代码段

$guest = "this is a string"; 
$guestArray = explode(" ",$guest); 
var_dump($guestArray); 
sort($guestArray); 
echo implode(",",$guestArray); 
---------------------输出结果---------------------------------------- 
array(4) { [0]=> string(4) "this" [1]=> string(2) "is" [2]=> string(1) "a" [3]=> string(6) "string" } a,is,string,this 
--------------------------------------------------------------------- 

5.替换字符和子字符串
string substr_replace(string source,string replace,int start[,int length])