What are the differences between passing by value and passing by reference in PHP?

Passing value means copying the value of a variable to a new value; within the function scope, changing the value of the variable will not affect the value of the variable outside the function. Passing by reference copies the reference of the variable; within the function scope, any changes to the value are also reflected outside the function, because passing by reference passes the memory address.

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php value passing: in function scope Changing the variable value inside the function will not affect the variable value outside the function.

PHP By reference: Within the function scope, any changes to the value are also reflected outside the function, because the memory address is passed by reference.

The difference between passing by value, passing by reference, and passing address:

1. Passing by value is to assign the value of the actual parameter to the line parameter

Then the modification of the row parameter will not affect the value of the actual parameter

2. Passing the address

is a special way of passing the value, but what it passes is the address, not the ordinary one. int

After passing the address, the actual parameters and line parameters point to the same object

3. Passing the reference

Really pass the parameters by address

After passing, the row parameters and actual parameters are the same object, but their names are different

Modification of the row parameters will affect the value of the actual parameters

Daniel's Explanation:

Pass value: It is the same as copy. [For example, I have a house. I give you building materials. You build a house that is exactly the same as mine. Whatever you do in your house will not affect me. What I do in my house will not affect me. Nothing will affect you, independent of each other. 】

<?php 
$testa=1; //定义变量a 
$testb=2; //定义变量b 
$testb = $testa; //变量a赋值给变量b 
echo $testb; //显示为1 
?>

Passing by reference: Similar to pointers in C language, it feels almost the same. For example, I have a house. If I give you a key, both of us can enter the house. Whatever you do in the house will affect me.

[Advantages and Disadvantages:] Passing values ​​will be time-consuming, especially for large strings and objects. This will be a very expensive operation. Transferring references is equivalent to any operation within the function. For operations on transfer variables, it is highly efficient when transferring large variables!

1. Let’s explain the noun first.

During the value transfer (passl-by-value) process, the formal parameters of the called function are treated as local variables of the called function, that is, a memory space is opened in the stack to store the parameters put in by the calling function. The value of the actual parameter becomes a copy of the actual parameter. The characteristic of value transfer is that any operation of the called function on the formal parameters is performed as a local variable and will not affect the value of the actual parameter variable of the calling function.

During the pass-by-reference process, although the formal parameters of the called function are also used as local variables to open up memory space on the stack, what is stored at this time is put in by the calling function. The address of the actual parameter variable. Any operation of the called function on the formal parameters is processed as indirect addressing, that is, the actual parameter variables in the calling function are accessed through the address stored in the stack. Because of this, any operation the called function does on the formal parameters affects the calling function.

Note: The red text above explains that application transfer does not open up space, but does open up space, but the space opened up is used to store the actual parameter variable address.

2.php has the following three usages.

①. Reference assignment of variables: $a = &$b

②. Reference parameter passing when calling function

1) In the early days of PHP, the & symbol was used when calling Pass reference type variables, such as: func(&$arg);

2) Later, the reference type parameters of functions were stipulated to need to be defined when the function is declared, instead of: function func(&$arg);

Note:After defining reference type parameters during reference declaration, runtime reference parameter passing is abandoned. You need to add allow_call_time_pass_reference to php.ini to enable it.

③. The function returns a reference type. This application method requires adding an & symbol before the function name when declaring the function, and when calling, the reference assignment method must be used. The example code is as follows:

function &func() {
    return $a;
}
$a = func();  //这种调用方式得到的不是引用传值
$a =& func(); //这样调用才是引用传值

$a = 1;
function &func(&$a) {
  return $a;
}
$b = func($a);
$c =& func($a);
$b = 2;
echo "a: $a, b: $b, c: $c. <br />/n";
//输出a: 1, b: 2, c: 1.
//可见对$b的修改不会影响$a
$c = 3;
echo "a: $a, b: $b, c: $c. <br />/n";
//输出a: 3, b: 2, c: 3.
//可见对$c的修改会影响$a

Please see the detailed example below ：

3.php Various data types pass value/pointer

1. Value pass of basic data types

/* **************************************************** */ 
function testvar($k){
 $k = 40;
}
$c = 30;
//给一个函数参数传一个基本数据类型(整型，布尔，字符 ...)， 实际上传的就是值 ;
testvar($c);
echo $c;//结果是：30
function testvar2(&$k){
 $k = 40;
}
$e = 30;
//给一个函数参数传一个基本数据类型(整型，布尔，字符 ...)， 实际上传的y就是地址 ;
testvar2($e);
echo $e;//结果是：40
  
/* **************************************************** */

2. Array (by default, a copy of the data is copied), if you want to pass the address, then &$arr

 1 $arr1 = array(-1,5,0); 
 2 function testArr($arr){ 
 3  for($i=0;$i<count ($arr);$i++){ 
 4   for($j=$i+1;$j<count($arr);$j++){ 
 5    if($arr[$i]>$arr[$j]){ 
 6     $temp = $arr[$i]; 
 7     $arr[$i] = $arr[$j]; 
 8     $arr[$j] = $temp; 
 9    } 
10   } 
11  
12  } 
13  print_r($arr);  //结果:Array ( [0] => -1 [1] => 0 [2] => 5 )  
14 } 
15 testArr($arr1); 
16 print_r($arr1);  //结果:Array ( [0] => -1 [1] => 5 [2] => 0 )  
17  
18 function testArr2(&$arr){ 
19  for($i=0;$i</count><count ($arr);$i++){ 
20   for($j=$i+1;$j<count($arr);$j++){ 
21    if($arr[$i]>$arr[$j]){ 
22     $temp = $arr[$i]; 
23     $arr[$i] = $arr[$j]; 
24     $arr[$j] = $temp; 
25    } 
26   } 
27  
28  } 
29 } 
30 testArr($arr1); 
31 print_r($arr1);  //结果:Array ( [0] => -1 [1] => 0 [2] => 5 )

3. Pass value of object data type

class person{
 public $name;
 public  $age;
}
  
$a = new person();
$a->name = '小明';
$a->age = '20';
//变量a在存的是对象的地址，把a赋给b这个变量，实际上就是赋了一个地址。
$b = $a;
$b->age = 30;
//echo $a->age.$b->age;//结果是：30 30
//给一个函数参数传一个对象， 实际上传的是这个对象的地址;
function test($k){
 $k->age =40;
}
//调用
test($b);
//echo $a->age.$b->age;//结果是：40 40

In PHP5, object copying is achieved by reference. In the above column, $a=new person; $b=$a; is actually equivalent to $a=new person; $b=&$a;
The default in PHP5 is to call objects by reference, but sometimes you may want to Create a copy of an object and hope that changes to the original object will not affect the copy. For this purpose, PHP defines a special method called __clone.

4.php copy-on-write

php中对于地址的指向（类似指针）功能不是由用户自己来实现的，是由Zend核心实现的，php中引用采用的是“写时拷贝”的原理，就是除非发生写操作，指向同一个地址的变量或者对象是不会被拷贝的。

通俗的讲

1:如果有下面的代码

$a="ABC";
 $b=$a;

其实此时$a与$b都是指向同一内存地址而并不是$a与$b占用不同的内存

２:如果在上面的代码基础上再加上如下代码

$a="EFG";

由于$a与$b所指向的内存的数据要重新写一次了，此时Zend核心会自动判断自动为$b生产一个$a的数据拷贝，重新申请一块内存进行存储。

5.php引用于C指针的区别

在PHP 中引用的意思是：不同的名字访问同一个变量内容.
与Ｃ语言中的指针是有差别的．Ｃ语言中的指针里面存储的是变量的内容在内存中存放的地址。

PHP 的引用允许你用两个变量来指向同一个内容

$a="ABC";
$b =&$a;
echo $a;//这里输出:ABC
echo $b;//这里输出:ABC
$b="EFG";
echo $a;//这里$a的值变为EFG 所以输出EFG
echo $b;//这里输出EFG

当你 unset 一个引用，只是断开了变量名和变量内容之间的绑定。这并不意味着变量内容被销毁了。例如：

unset($a);
echo $b;//这里输出EFG

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