mixed str_replace ( mixed $search , mixed $replace , mixed $subject [, int &$count ] )
该函数返回一个字符串或者数组。该字符串或数组是将subject中全部的search都被replace替换之后的结果。
1、$search,要替换的字符串，或数组
2、$replace,被用来替换的字符串或数组
3、$subject,被查询的字符串或数组
4、$count，可选，如果被指定，将为设置为替换的次数
5、返回值：该函数返回替换后的数组或者字符串(新生成的)

<?php
    //实例一：字符串替换字符串
    $str1 = str_replace("red","black","red green yellow pink purple");
    echo $str1."";    //输出结果为black green yellow pink purple
?>

<?php
    //实例二：字符串替换数组键值
    $arr = array("blue","red","green","yellow");
    $str1 = str_replace("red","pink",$arr,$i);
    print_r($str1);
?>

<?php
    //实例三：数组替换数组，映射替换
    $arr1 = array("banana","orange");
    $arr2 = array("pitaya","tomato");
    $con_arr = array("apple","orange","banana","grape");
    $con_rep = str_replace($arr1,$arr2,$con_arr,$count);
    print_r($con_rep);
?>

<?php
    //实例四：如$search为数组，$replace为字符串时
    $search = array("banana","grape");
    $replace = "tomato";
    $arr = array("banana","apple","orange","grape");
    $new_arr = str_replace($search,$replace,$arr,$count);
    print_r($new_arr);
?>

```