Detailed explanation of PHP reference variable knowledge

This article mainly introduces the detailed knowledge about PHP reference variables. It has a certain reference value. Now I share it with everyone. Friends in need can refer to it

• Concept: Reference in PHP means accessing the same variable content with different names;

• Definition method: PHP uses '& 'Define reference variables;

• #When references are not used, variables adopt a copy-on-write mechanism (COW): a copy of the memory will be copied for modification when writing, for example

//定义一个变量
$a = range(0,1000);
var_dump(memory_get_usage()); //打印内存使用量
//定义一个变量b
$b = $a; //此时$b和$a 指向同一个内存空间
var_dump(memory_get_usage());
//当a或b发生写入时，才会复制一块内存进行修改
$a = range(0,1000);
var_dump(memory_get_usage());

Running results: The memory usage was similar for the first and second printing times, but there was a significant change for the third time, indicating that when a performed a write operation, the memory usage occurred copy.

• #When using a reference, the variable will point to the same memory space and will not change during the write operation.

//定义一个变量
$a = range(0,1000);
var_dump(memory_get_usage()); //打印内存使用量
//定义一个变量b
$b = &$a; //将a的空间赋值给b，a与b指向同一空间
var_dump(memory_get_usage());
//当a或b发生写入时，内存不会发生复制
$a = range(0,1000);
var_dump(memory_get_usage());

Running results: The memory has not changed significantly

##Verified through the zval variable container

//通过zval变量容器打印
$a = range(0,3);
xdebug_debug_zval('a');//打印 指向内存空间的变量数，和是否被引用
$c =&$a;
xdebug_debug_zval('a');
$c = range(0,3);
xdebug_debug_zval('a');

Running results:

When writing to c, there are still 2 variables pointing to the memory, and copy-on-write does not occur

##unset will only cancel the reference and will not destroy the space
In PHP, the object itself is passed by reference and does not need to use reference symbols.

• class Person{
      public $name  = "zhangsan";
  }
  $p1 = new Person();
  xdebug_debug_zval('p1');
  
  $p2 = $p1;
  xdebug_debug_zval('p1');
  
  $p2->name = "lesi";
  xdebug_debug_zval('p1');

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