How are PHP variables defined and how does PHP work?

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1. Variable definition:

What we usually learn is that a variable represents an "identifier" for the storage space and the data in it.

The variable name points to the variable value

More deeply, the variable points to an area of ​​memory

2. The working principle of variables, through drawing analysis method-memory space

<?php
//php引用变量：不同变量名指向同一地址。
//定义一个变量a，此时内存开辟了一块区域，$a指向该区域。
$a = 100;
var_dump($a);    //int 100
//定义变量b，将a变量的值赋值给b，此时该区域有两个变量（$a和$b）指向。
$b = $a;
var_dump($b);    //int 100
//修改$a变量的值，php变量具有Copy On Write的特性，所以会复制并重写a所指向的区域值，此时a和b分别指向不同区域。
$a = 10;
var_dump($a);    //int 10
var_dump($b);    //int 100
//------------------------------------------------------------------
$c = 100;
$d = &$c;
$c = 10;
var_dump($c);    //int 10
var_dump($d);    //int 10
//当引用指向时，php变量不在具有COW特性，且指向同一内存区域。
//* unset()只能消除变量的引用，不能删除其内存分配的空间
//------------------------------------------------------------------
/*
例题：
    写出如下程序的输出结果：
    <?php
    $data = [&#39;a&#39;,&#39;b&#39;,&#39;c&#39;];
    foreach($data as $key=>$val){
        $val = &$data[$key];
    }
    问：最终$data的值是多少？
 */
    $data = [&#39;a&#39;,&#39;b&#39;,&#39;c&#39;];
    foreach($data as $key=>$val){
        $val = &$data[$key];
        print_r($data);
    }
/*
    1.$key = 0,
      $val = &#39;a&#39;,
      $val = &$d[0] =>&#39;a&#39;
      $data = [&#39;a&#39;,&#39;b&#39;,&#39;c&#39;];
2.$key = 1,
      $val = &#39;b&#39;, =>$d[0]
      $val = &$d[1] =>&#39;b&#39;
      $data = [&#39;b&#39;,&#39;b&#39;,&#39;c&#39;];
1.$key = 2,
      $val = &#39;c&#39;, =>$d[1]
      $val = &$d[2]=>&#39;c&#39;
      $data = [&#39;b&#39;,&#39;c&#39;,&#39;c&#39;];
 */

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