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：
<div class="codetitle"><a style="CURSOR: pointer" data="75020" class="copybut" id="copybut75020" onclick="doCopy('code75020')"> 代码如下: