Solution to error when accessing array elements in PHP double quotes

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Recently I am developing a WeChat public account. In an interface for sending graphics and text, I need to splice array elements into XMLString中

foreach ($itemArr as $key => $value){ 
  $items .= "<item> 
  <Title><![CDATA[$value[&#39;title&#39;]]]></Title>  
  <Description><![CDATA[[$value[&#39;description&#39;]]]></Description> 
  <PicUrl><![CDATA[$value[&#39;picUrl&#39;]]]></PicUrl> 
  <Url><![CDATA[$value[&#39;url&#39;]]]></Url> 
  </item>"; 
}

The result is reported as followsError message:

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

From the error message, it seems that it is a single quote Problem, after decisively removing it, no error was reported. However, I am confused. Shouldn't quotes be added to array elements whose subscripts are strings? I went to the official PHP manual to check the description of arrays. There is a paragraph like this:

$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']";

There are two wrong ways of writing, when an ordinary array variable or super global array variable is contained in double quotes When referencing an array element whose index is a string, single quotes should not be added to the index string. So what is the correct way to write it? So I continued to search the official manual and found the following statement:

$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

Here are three correct ways to write:

First methodWrite the index string without adding any quotation marks. At this time, it means to get the array element whose index is the string fruit and output apple.

The second way of writing the index string is without adding any quotation marks, and at the same time, the array variable is wrapped in a pair of curly braces { }. At this time, fruit It actually represents a constant, not a string, so it means getting the array element whose index is the fruit constant value. The value of the constant fruit is veggie, so carrot is output.

The third way of writing is to quote the string by not only adding single quotes, but also wrapping the array variable with a pair of curly braces { }. This way When it means to get the array element whose index is the string fruit, output apple. Later I continued to search and found this piece of code:

// 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[&#39;fruit&#39;]; // 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

Under normal circumstances, when the array variable is not surrounded by double quotes, the output results will be consistent whether single quotes are added to the index string. It is apple, but when a constant with the same name as the index string fruit is defined, the output result of the index string without single quotes becomes carrot, but with single quotes it is still apple.

Conclusion:

1. When the array variable is not enclosed in double quotes, (1) The index string with single quotes represents the string itself

<pre name="code" class="php">$arr[&#39;fruit&#39;]

(2) The index string without single quotes represents a constant. When the constant is undefined, it is parsed into a string, etc. Effective by adding single quotes.

$arr[fruit]

2. When the array variable is enclosed in double quotes, (1) The index string represents the string itself without single quotes

"$arr[fruit]"

(2) Adding curly brackets to an array variable indicates a constant with the same name as a string

"{$arr[fruit]}"

(3) Adding single quotes to an index string and adding curly brackets to an array variable indicates the string itself

<pre name="code" class="php"><pre name="code" class="php">"{$arr[&#39;fruit&#39;]}"

<pre name="code" class="php"><pre name="code" class="php">"$arr[&#39;fruit&#39;]"

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