在最近的项目中，会碰到到某人的手机号码隐藏中间几位，身份证号码只显示末尾4位的需求。当时一开始是网上搜索了一下，看到有人是用substr_replace这个函数来替换的，后面我也用了这个函数，但在用的时候不是很好用。

一、substr_replace

先来看看这个函数的语法：

substr_replace(string,replacement,start,length)

1、当start与charlist都为正数的时候，非常好理解，也很符号人的逻辑，start是从0开始的，如下图，根据条件，绿色的将是要被替换的元素

aaarticlea/png;base64,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" alt="" />

2、当start为负数，charlist为正数的时候，也挺好理解的

aaarticlea/png;base64,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" alt="" />

3、当start为正数，charlist为负数的时候，这个我一开始理解错了

aaarticlea/png;base64,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" alt="" />

4、当start为负数，charlist为负数的时候，有一个地方需要注意的就是：如果 start 是负数且 length 小于等于 start，则 length 为 0。这个坑挺容易踩到的

aaarticlea/png;base64,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" alt="" />

5、charlist为0的时候，就变成插入了，而不是替换，额。。。

用下来，我是感觉不是很顺手，虽然说满足我现在的需求还是可以的，但是如果将来需要一些扩展的话，耍起来挺吃力的，所以就想到自己构造一个，将来用起来也方便。

二、自制的星号替换函数

replaceStar($str, $start, $length = 0)

前面的两个参数与上面的一样，最后的参数与上面不同

1、当start与length都为正数，与substr_replace表现的一样

2、当start为负数，length为正数，与substr_replace表现的一样

substr_replace

replaceStar

三、源码分享

public static function replaceStar($str, $start, $length = 0)

{

        $i = 0;

        $star = '';

        if($start >= 0) {

            if($length > 0) {

                $str_len = strlen($str);

                $count = $length;

                if($start >= $str_len) {//当开始的下标大于字符串长度的时候，就不做替换了

                    $count = 0;

                }

            }elseif($length < 0){

                $str_len = strlen($str);

                $count = abs($length);

                if($start >= $str_len) {//当开始的下标大于字符串长度的时候，由于是反向的，就从最后那个字符的下标开始

                    $start = $str_len - 1;

                }

                $offset = $start - $count + 1;//起点下标减去数量，计算偏移量

                $count = $offset >= 0 ? abs($length) : ($start + 1);//偏移量大于等于0说明没有超过最左边，小于0了说明超过了最左边，就用起点到最左边的长度

                $start = $offset >= 0 ? $offset : 0;//从最左边或左边的某个位置开始

            }else {

                $str_len = strlen($str);

                $count = $str_len - $start;//计算要替换的数量

            }

        }else {

            if($length > 0) {

                $offset = abs($start);

                $count = $offset >= $length ? $length : $offset;//大于等于长度的时候 没有超出最右边

            }elseif($length < 0){

                $str_len = strlen($str);

                $end = $str_len + $start;//计算偏移的结尾值

                $offset = abs($start + $length) - 1;//计算偏移量，由于都是负数就加起来

                $start = $str_len - $offset;//计算起点值

                $start = $start >= 0 ? $start : 0;

                $count = $end - $start + 1;

            }else {

                $str_len = strlen($str);

                $count = $str_len + $start + 1;//计算需要偏移的长度

                $start = 0;

            }

        }

        while ($i < $count) {

            $star .= '*';

            $i++;

        }

        return substr_replace($str, $star, $start, $count);

}

不擅长算法，这里就用很普通的逻辑来展示啦，没有用到啥数学公式。

1、if($start >= 0)这里做start大于等于0与小于0的分支

2、在start 的分之中，分别再做length 大于0，小于0和等于0的三个分支

3、最后计算出start、count和要替换的星号字符串，最后计算出的start与count都是正数，运用substr_replace做替换

四、单元测试

public function testReplaceStar()

    {

        $actual = App_Util_String::replaceStar('123456789', 3, 2);

        $this->assertEquals($actual, '123**6789');

        $actual = App_Util_String::replaceStar('123456789', 9);

        $this->assertEquals($actual, '123456789');

        $actual = App_Util_String::replaceStar('123456789', 9, 2);

        $this->assertEquals($actual, '123456789');

        $actual = App_Util_String::replaceStar('123456789', 9, -9);

        $this->assertEquals($actual, '*********');

        $actual = App_Util_String::replaceStar('123456789', 9, -10);

        $this->assertEquals($actual, '*********');

        $actual = App_Util_String::replaceStar('123456789', 9, -11);

        $this->assertEquals($actual, '*********');

        $actual = App_Util_String::replaceStar('123456789', 3);

        $this->assertEquals($actual, '123******');

        $actual = App_Util_String::replaceStar('123456789', 0);

        $this->assertEquals($actual, '*********');

        $actual = App_Util_String::replaceStar('123456789', 0, 2);

        $this->assertEquals($actual, '**3456789');

        $actual = App_Util_String::replaceStar('123456789', 3, -3);

        $this->assertEquals($actual, '1***56789');

        $actual = App_Util_String::replaceStar('123456789', 1, -5);

        $this->assertEquals($actual, '**3456789');

        $actual = App_Util_String::replaceStar('123456789', 3, -3);

        $this->assertEquals($actual, '1***56789');

        $actual = App_Util_String::replaceStar('123456789', -3, 2);

        $this->assertEquals($actual, '123456**9');

        $actual = App_Util_String::replaceStar('123456789', -3, 5);

        $this->assertEquals($actual, '123456***');

        $actual = App_Util_String::replaceStar('123456789', -1, 2);

        $this->assertEquals($actual, '12345678*');

        $actual = App_Util_String::replaceStar('123456789', -1, -2);

        $this->assertEquals($actual, '1234567**');

        $actual = App_Util_String::replaceStar('123456789', -4, -7);

        $this->assertEquals($actual, '******789');

        $actual = App_Util_String::replaceStar('123456789', -1, -3);

        $this->assertEquals($actual, '123456***');

        $actual = App_Util_String::replaceStar('123456789', -1);

        $this->assertEquals($actual, '*********');

        $actual = App_Util_String::replaceStar('123456789', -2);

        $this->assertEquals($actual, '********9');

        $actual = App_Util_String::replaceStar('123456789', -9);

        $this->assertEquals($actual, '*23456789');

        $actual = App_Util_String::replaceStar('123456789', -10);

        $this->assertEquals($actual, '123456789');

        $actual = App_Util_String::replaceStar('123456789', -10, -2);

        $this->assertEquals($actual, '123456789');

    }