函数原型：mixed str_replace ( mixed $search , mixed $replace , mixed $subject [, int &$count ] )
这个函数的三个参数类型都是mixed，所以它们都既可以是字符串，也可以是数组，因此就有以下几种用法：
1、 $search和$replace都是字符串，这是最常见的用法
echo str_replace("a", "apple", "This is a demo.");输出：This is apple demo.
2、$search为数组，对$search中的数组元素逐个替换成$replace，最终输出为字符串。
echo str_replace(array("a", "p","o"), "apple", "This is a demo.");输出：This is aappleapplele demapple.
3、$replace为数组，会报错且不会输出预想的结果，不建议使用
echo str_replace("a", array("apple", "pear"), "This is a demo.");输出：This is Array demo.并报错 Notice: Array to string conversion
4、$subject为数组，$subject中的每个数组元素都会做一次独立替换，最终输出为数组。
print_r(str_replace("a", "apple", array("This is a demo.", "This is not a demo.")));输出：Array ( [0] => This is apple demo. [1] => This is not apple demo. )
5、$search和$replace都为数组，这时又可以分为三种情况：
a）.$search和$replace一样长，则相同下标对应替换
b）.$search比$replace长，则$search中比$replace多出的数组元素都被替换成空字符串
c）.$search比$replace短，则$replace中多出的数组元素被忽略
echo str_replace(array("a", "o"), array("apple", "pear"), "This is a demo.");输出：This is apple dempear.
6、$search、$replace和$subject都为数组，这是以上多种情况的综合
print_r(str_replace(array("a", "o"), array("apple", "pear"), array("This is a demo.", "This is not a demo.")));输出：Array ( [0] => This is apple dempear. [1] => This is npeart apple dempear. )
