Detailed explanation of PHP syntax rules

1. Simple syntax rules (delimit variable names with braces, applicable to all versions of PHP):

$a = 'flower';
echo "She received some $as";
// 无效；字母s会被当成有效的变量名组成元素，但是这里的变量是$a
echo "She received some ${a}s"; // 有效
echo "She received some {$a}s"; // 有效；推荐的使用方法

What we want to express is "she received some flowers", and the word "flower" in the context should be in the plural form (also That is to say, S) should be added at the end, but if the variables are not defined in any way, the first echo will occur. Obviously we want the output to be $a rather than $as. So how do we usually process this output?

echo "She received some $a"."s"; 
echo "She received some ".$a."s";

These two customary ways of writing should be less concise and clear than the way of writing with curly brackets, right?
Note: No matter whether { appears before or after $, curly braces will be regarded as delimiting symbols only when the two are immediately adjacent. Do not add spaces between them, otherwise they will be treated as ordinary curly braces

echo "She received some { $a}s"; 
// 输出的结果为：She received some { flower}s

2. Complex syntax rules (using curly braces to define expressions, etc., used with PHP4+):

echo "有效的写法： {$arr[4][3]}";
// 有效；界定多维数组
echo "有效的写法： {$arr['foo'][3]}";
// 有效；当在字符串中使用多维数组时，一定要用括号将它括起来
echo "有效的写法： {$this->width}00"; 
// 有效；如果不界定的话，就会变成 $this->width00
echo "有效的写法： {$this->value[3]->name}";  
// 有效；该例演示了界定链式调用
echo "有效的写法： $name: {${$name}}"; 
// 有效；该例演示的效果实际上是一个可变变量
echo "有效的写法: {${getName()}}"; 
// 有效；该例演示了将函数的返回值作为变量名
echo "有效的下发： {${$this->getName()}}"; 
// 有效；该例演示了将函数的返回值作为变量名
注意1：echo "这样写有效吗： {getName()}";输出结果为：'这样写有效吗： 
{getName()}'。因为里面不含$，所以花括号不会被当作界定符
注意2：echo "这样写有效吗：{$arr[foo][3]}"; 在回答这个问题前我们先来进行一个实验：
error_reporting(E_ALL);
$arr = array('a', 'b', 'c', 'd'=>'e');
echo "This is $arr[d]";
// 我们发现这样写是没有问题的，那么我们像下面这样写呢？
echo $arr[d];

produced such an error:
Notice: Use of undefined constant d - assumed 'd'
Note: An undefined constant d is used, which should probably be 'd'
So if we modify the code as follows

error_reporting(E_ALL);
$arr = array('a', 'b', 'c', 'd'=>'e');
define('f', 'd');
echo $arr[f];

We find that there is no problem this time . It can be seen that there is no problem if the index of the array in the string is not enclosed in single quotes, but if this writing method does not appear in the string, an error will be reported, and for {$arr[foo][3]} in the string Parsing is based on non-string parsing. Therefore, it is wrong to only add curly braces to delimit the array in the string without adding single quotes to the index. Because the program will parse the index without single quotes as a constant, this generates an error. The correct way of writing should be:
echo "Effective way of writing: {$arr['foo'][3]}";
A special reminder: echo "This is $arr[d]"; Although this way of writing can be used by the program parsing, but this is limited to the case where the array is a one-dimensional array. The rigorous writing method should be: echo "This is {$arr['d']}"; My student once argued with me on this point. He said: Since the previous writing method can produce results, why must we use it? What about the latter way of writing? So, let’s continue to modify the previous code

error_reporting(E_ALL);
$arr = array('a', 'b', 'c', 
'd'=>array('e'=>'f')
);
echo "This is $arr[d][e]";

Can it still be parsed correctly? I just want to tell you that adding curly braces is strictly necessary. Of course, if you are not my student, then I can’t control that much...

Note 3:

error_reporting(E_ALL);  
$arr = array('a', 'b', 'c', 'd');  
echo "This is {$arr[2]} <br />";  
echo "This is {$arr[&#39;2&#39;]} <br />";

Execute the above code. The result is the same, why is this? I can only tell you that PHP is a weakly typed language. As for what a weakly typed language is, I won’t say more here. Go Google it yourself. Having said so much, where are the specific applications that best reflect the advantages of these syntactic rules? ----SQL statement

Java code

// 示例一：  
$SQL1 = "select * from table where id={$_GET['id']}";  
// 示例二：  
$SQL2 = "select * from table where id={$this->id}";