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 中的原話。
需要注意的時(shí)候，我們訪問數(shù)組的時(shí)候 都是使用方括號(hào)“[]”，string作為一個(gè)也可以使用操作符“[]”進(jìn)行訪問。但是，需要注意的一點(diǎn)就是，訪問字符串時(shí)候,操作符“[]”中的內(nèi)容會(huì)被轉(zhuǎn)化為int類型的。
eg: $str ='123456';
echo $str['php'];//結(jié)果是1,因?yàn)閛ffset ‘php'轉(zhuǎn)化為integer為0，既是訪問的是字符串的第一個(gè)字符.
var_dump(isset($str['php']));//結(jié)果是bool(true) 原理同上。
所以，在我們使用isset判斷一個(gè)設(shè)置是否存在某個(gè)鍵時(shí)候，應(yīng)該先判斷試下，傳遞過來的變量是否是數(shù)組，然后再判斷是否是存在指定的key
eg：//如果需要判斷傳遞過來的數(shù)組是否存在'php'這個(gè)key時(shí)候，比較安全的做法為：

如果 上面的函數(shù) 沒有添加 is_array 的判斷，當(dāng)傳遞一個(gè) 字符串過來的時(shí)候， 結(jié)果就不是我們預(yù)想的那樣了。

僅此為記，以免以后也出現(xiàn)類似的問題。

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