最近在做微信公眾號開發(fā)，在一個發(fā)送圖文接口中，需要把數(shù)組元素拼接在XML字符串中
foreach ($itemArr as $key => $value){
$items .= "<item>
<Title><![CDATA[$value['title']]]></Title>
<Description><![CDATA[[$value['description']]]></Description>
<PicUrl><![CDATA[$value['picUrl']]]></PicUrl>
<Url><![CDATA[$value['url']]]></Url>
</item>";
}
結(jié)果竟報如下錯誤信息：
Parse error: syntax error, unexpected '' (T_ENCAPSED_AND_WHITESPACE), expecting identifier (T_STRING) or variable (T_VARIABLE) or number (T_NUM_STRING) in D:\hhp\wamp\www\weixin\wx_sample.php on line 146
從錯誤信息看是單引號的問題，果斷去掉之后就沒報錯了。然而我就納悶了，引用下標為字符串的數(shù)組元素難道不該加引號嗎？到php官方手冊去查了關(guān)于數(shù)組的描述，有一段是這樣的：
$arr = array('fruit' => 'apple', 'veggie' => 'carrot');
// This will not work, and will result in a parse error, such as:
// Parse error: parse error, expecting T_STRING' or T_VARIABLE' or T_NUM_STRING'
// This of course applies to using superglobals in strings as well
print "Hello $arr['fruit']";
print "Hello $_GET['foo']";
這里給出了兩種錯誤的寫法，當一個普通數(shù)組變量或超全局數(shù)組變量包含在雙引號中時，引用索引為字符串的數(shù)組元素，索引字符串不應(yīng)該再添加單引號。那正確的寫法是怎樣的呢？于是我繼續(xù)查找官方手冊，找到如下說法：
$arr = array('fruit' => 'apple', 'veggie' => 'carrot');
// This defines a constant to demonstrate what's going on. The value 'veggie'
// is assigned to a constant named fruit.
define('fruit', 'veggie');
// The following is okay, as it's inside a string. Constants are not looked for// within strings, so no E_NOTICE occurs hereprint "Hello $arr[fruit]"; // Hello apple// With one exception: braces surrounding arrays within strings allows constants// to be interpretedprint "Hello {$arr[fruit]}"; // Hello carrotprint "Hello {$arr['fruit']}"; // Hello apple
$arr = array('fruit' => 'apple', 'veggie' => 'carrot');
// This defines a constant to demonstrate what's going on. The value 'veggie'
// is assigned to a constant named fruit.
define('fruit', 'veggie');
// The following is okay, as it's inside a string. Constants are not looked for
// within strings, so no E_NOTICE occurs here
print "Hello $arr[fruit]"; // Hello apple
// With one exception: braces surrounding arrays within strings allows constants
// to be interpreted
print "Hello {$arr[fruit]}"; // Hello carrot
print "Hello {$arr['fruit']}"; // Hello apple
這里給出了三種正確的寫法：
第一種寫法索引字符串不添加任何引號，此時表示獲取索引為字符串fruit的數(shù)組元素，輸出apple。
第二種寫法索引字符串也沒有添加任何引號，同時將數(shù)組變量用一對花括號{ }給包了起來，此時fruit實際上表示一個常量，而不是一個字符串，因此表示獲取索引為fruit常量值的數(shù)組元素，常量fruit的值是veggie,所以輸出carrot。
第三種寫法是引用字符串不但添加了單引號，同時也將數(shù)組變量用一對花括號{ }給包了起來，此時表示獲取索引為字符串fruit的數(shù)組元素，輸出apple。
后來我繼續(xù)查找，發(fā)現(xiàn)這樣一段代碼：
// Incorrect. This works but also throws a PHP error of level E_NOTICE because // of an undefined constant named fruit // // Notice: Use of undefined constant fruit - assumed 'fruit' in... print $arr[fruit]; // apple <pre name="code" class="php">print $arr['fruit']; // apple
// This defines a constant to demonstrate what's going on. The value 'veggie'// is assigned to a constant named fruit.define('fruit', 'veggie');// Notice the difference nowprint $arr[fruit]; // carrot
print $arr['fruit']; // apple
在正常情況下，數(shù)組變量沒有被雙引號包圍時，是否給索引字符串加上單引號輸出結(jié)果都一致時apple，但是當定義一個與索引字符串fruit同名的常量時，未加單引號的索引字符串輸出結(jié)果就成了carrot，而加上單引號還是apple。
結(jié)論：
1. 數(shù)組變量未用雙引號包括時，
(1) 索引字符串加單引號表示字符串本身
<pre name="code" class="php">$arr['fruit']