PHP：字符串转数组，数组转字符串；字符串截取、替换、查找

字符串转数组
$str = 'one|two|three|four';
print_r(explode('|', $str));   //explode 以字符串分割字符串到数组

$str = 'one two three';       //str_split 以位数分割
$arr1 = str_split($str);
$arr2 = str_split($str, 3);
print_r($arr1);
print_r($arr2);

打印如下：
Array
(
    [0] => o
    [1] => n
    [2] => e
    [3] =>  
    [4] => t
    [5] => w
    [6] => o
    [7] =>  
    [8] => t
    [9] => h
    [10] => r
    [11] => e
    [12] => e
)
Array
(
    [0] => one
    [1] =>  tw
    [2] => o t
    [3] => hre
    [4] => e
)

数组转字符串
inplode()
字符串截取
substring 方法用于提取字符串中介于两个指定下标之间的字符
substring(start,end)
substr 方法用于返回一个从指定位置开始的指定长度的子字符串。
substr(start [, length ])
替换
str_replace()
把字符串 "Hello world!" 中的字符 "world" 替换为 "Shanghai"：

echo str_replace("world","Shanghai","Hello world!");

preg_replace:执行一个正则表达式的搜索和替换

查找
preg_match();
preg_grep();