How to solve the error when accessing array elements in PHP double quotes

Recently I am developing a WeChat public account. In an interface for sending graphics and text, array elements need to be spliced ​​into XML strings. This article mainly shares with you a solution to errors when accessing array elements in PHP double quotes. It has a very good reference value and I hope it will be helpful to everyone. Let’s follow the editor to take a look, I hope it can help everyone.

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>"; 
}

As a result, the following error message was reported:

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

Judging from the error message, it was a problem with single quotes. 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

There are three correct ways of writing:

The first way of writing the index string does not add any quotation marks, which means getting The array element whose index is the string fruit, outputs apple.

The second way of writing the index string does not add any quotes, and at the same time wraps the array variable with a pair of curly braces { }. At this time, fruit actually represents a constant, not a string. Therefore, 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 means getting 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['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

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) Add single quotes to the index string to represent the string itself

<pre name="code" class="php">$arr['fruit']

(2) The index string without single quotes represents a constant. When the constant is not defined, it is parsed into a string, which is equivalent to adding single quotes.

$arr[fruit]

2. When the array variable is enclosed in double quotes,

(1) The index string does not include single quotes to represent the string itself

"$arr[fruit]"

(2) The array variable includes The upper curly braces represent the constant with the same name as the string

"{$arr[fruit]}"

(3) The index string plus single quotes and the array variable plus curly braces represent the string itself

<pre name="code" class="php"><pre name="code" class="php">"{$arr['fruit']}"

(4) The index string Adding single quotes and not adding curly braces to the array variable is an incorrect way of writing. The error is reported: Parse error: parse error, expecting T_STRING' or T_VARIABLE' or T_NUM_STRING'

<pre name="code" class="php"><pre name="code" class="php">"$arr['fruit']"

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