PHP STRING Trap Principle Explanation_PHP Tutorial

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Release: 2016-07-21 15:35:25
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A string is series of characters.
String access and modification by character
Characters within strings may be accessed and modified by specifying the zero-based offset of the desired character after the string using square array brackets, as in $str[42]. Think of a string as an array of characters for this purpose.
Note: Strings may also be accessed using braces, as in $str{42}, for the same purpose. However, this syntax is deprecated as of PHP 5.3.0. Use square brackets instead, such as $str[42].
Warning
Writing to an out of range offset pads the string with spaces. Non-integer types are converted to integer. Illegal offset type emits E_NOTICE. Negative offset emits E_NOTICE in write but reads empty string. Only the first character of an assigned string is used. Assigning empty string assigns NUL byte.

以上都是 php manual 中的原话。
需要注意的时候,我们访问数组的时候 都是使用方括号“[]”,string作为一个也可以使用操作符“[]”进行访问。但是,需要注意的一点就是,访问字符串时候,操作符“[]”中的内容会被转化为int类型的。
eg: $str ='123456';
echo $str['php'];//结果是1,因为offset ‘php'转化为integer为0,既是访问的是字符串的第一个字符.
var_dump(isset($str['php']));//结果是bool(true) 原理同上。
所以,在我们使用isset判断一个设置是否存在某个键时候,应该先判断试下,传递过来的变量是否是数组,然后再判断是否是存在指定的key
eg://如果需要判断传递过来的数组是否存在'php'这个key时候,比较安全的做法为:

复制代码 代码如下:

function is_set($arr, $key){
if (is_array($arr) && isset($arr[$key])) {
//存在该值的逻辑
} else{
//$arr不是数组 或者 数组$arr不存在key $key的逻辑
}
}

如果 上面的函数 没有添加 is_array 的判断,当传递一个 字符串过来的时候, 结果就不是我们预想的那样了。

仅此为记,以免以后也出现类似的问题。

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