Detailed explanation of PHP syntax rules, detailed explanation of PHP syntax_PHP tutorial

Detailed explanation of PHP syntax rules, detailed explanation of PHP syntax

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

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

What we want to express is "she received some flowers". Flower in the context should be in the plural form (that is to say, S should be added after it), but if it is not used in the variable If any definition is made, the first echo situation will occur. Obviously we want the output to be $a rather than $as. So how do we usually process this output?

<span class="pln">echo <span class="str">"She received some $a"<span class="pun">.<span class="str">"s"<span class="pun">;<span class="pln"> 
echo <span class="str">"She received some "<span class="pun">.<span class="pln">$a<span class="pun">.<span class="str">"s"<span class="pun">;<span class="com">// 这两种习惯性的写法应该没有加花括号的写法简洁明了吧？</span></span></span></span></span></span></span></span></span></span></span></span></span>

Note: No matter whether { appears before or after $, curly braces will be regarded as delimiting symbols only when they are immediately adjacent. Do not add spaces between them, otherwise they will be treated as ordinary curly braces

<span class="pln">echo <span class="str">"She received some { $a}s"<span class="pun">;<span class="com">// 输出的结果为：She received some { flower}s</span></span></span></span>

2. Complex syntax rules (delimit expressions with braces, etc., used with PHP4):

<span class="pln">echo <span class="str">"有效的写法： {$arr[4][3]}"<span class="pun">;<span class="com">// 有效；界定多维数组<span class="pln">
echo <span class="str">"有效的写法： {$arr['foo'][3]}"<span class="pun">;<span class="com">// 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<span class="pln">
echo <span class="str">"有效的写法： {$this->width}00"<span class="pun">;<span class="com">// 有效；如果不界定的话，就会变成 $this->width00<span class="pln">
echo <span class="str">"有效的写法： {$this->value[3]->name}"<span class="pun">;<span class="com">// 有效；该例演示了界定链式调用<span class="pln">
echo <span class="str">"有效的写法： $name: {${$name}}"<span class="pun">;<span class="com">// 有效；该例演示的效果实际上是一个可变变量<span class="pln">
echo <span class="str">"有效的写法: {${getName()}}"<span class="pun">;<span class="com">// 有效；该例演示了将函数的返回值作为变量名<span class="pln">
echo <span class="str">"有效的下发： {${$this->getName()}}"<span class="pun">;<span class="com">// 有效；该例演示了将函数的返回值作为变量名</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span>

Note 1: echo "Is it valid to write like this: {getName()}"; the output result is: 'Is it valid to write like this:
{getName()}'. Because there is no $ in it, the curly braces will not be used as delimiters
Note 2: echo "Is it valid to write like this: {$arr[foo][3]}"; Before answering this question, we Let’s conduct an experiment first:

<span class="pln">error_reporting<span class="pun">(<span class="pln">E_ALL<span class="pun">);<span class="pln">
$arr <span class="pun">=<span class="pln"> array<span class="pun">(<span class="str">'a'<span class="pun">,<span class="str">'b'<span class="pun">,<span class="str">'c'<span class="pun">,<span class="str">'d'<span class="pun">=><span class="str">'e'<span class="pun">);<span class="pln">
echo <span class="str">"This is $arr[d]"<span class="pun">;<span class="com">// 我们发现这样写是没有问题的，那么我们像下面这样写呢？<span class="pln">
echo $arr<span class="pun">[<span class="pln">d<span class="pun">];</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span>

produced this 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

<span class="pln">error_reporting<span class="pun">(<span class="pln">E_ALL<span class="pun">);<span class="pln">
$arr <span class="pun">=<span class="pln"> array<span class="pun">(<span class="str">'a'<span class="pun">,<span class="str">'b'<span class="pun">,<span class="str">'c'<span class="pun">,<span class="str">'d'<span class="pun">=><span class="str">'e'<span class="pun">);<span class="pln">
define<span class="pun">(<span class="str">'f'<span class="pun">,<span class="str">'d'<span class="pun">);<span class="pln">
echo $arr<span class="pun">[<span class="pln">f<span class="pun">];</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span>

We found that there was 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 interprets unquoted indexes as constants, an error occurs. The correct writing should be:

<span class="pln">echo <span class="str">"有效的写法： {$arr['foo'][3]}"<span class="pun">;</span></span></span>

A special reminder: echo "This is $arr[d]"; although this writing method can be parsed by the program, it It is also limited to the case where the array is a one-dimensional array. The rigorous writing method should be: echo "This is {$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? Then, let’s continue to modify the previous code

<span class="pln">error_reporting<span class="pun">(<span class="pln">E_ALL<span class="pun">);<span class="pln">
$arr <span class="pun">=<span class="pln"> array<span class="pun">(<span class="str">'a'<span class="pun">,<span class="str">'b'<span class="pun">,<span class="str">'c'<span class="pun">,<span class="str">'d'<span class="pun">=><span class="pln">array<span class="pun">(<span class="str">'e'<span class="pun">=><span class="str">'f'<span class="pun">)<span class="pun">);<span class="pln">
echo <span class="str">"This is $arr[d][e]"<span class="pun">;</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span>

Can this 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:

<span class="pln">error_reporting<span class="pun">(<span class="pln">E_ALL<span class="pun">);<span class="pln">
$arr <span class="pun">=<span class="pln"> array<span class="pun">(<span class="str">'a'<span class="pun">,<span class="str">'b'<span class="pun">,<span class="str">'c'<span class="pun">,<span class="str">'d'<span class="pun">);<span class="pln">
echo <span class="str">"This is {$arr[2]} <br />"<span class="pun">;<span class="pln">
echo <span class="str">"This is {$arr['2']} <br />"<span class="pun">;</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span>

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

<span class="com">// 示例一：<span class="pln">
$SQL1 <span class="pun">=<span class="str">"select * from table where id={$_GET['id']}"<span class="pun">;<span class="com">// 示例二：<span class="pln">
$SQL2 <span class="pun">=<span class="str">"select * from table where id={$this->id}"<span class="pun">;</span></span></span></span></span></span></span></span></span></span>

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