Detailed explanation of the use of reference symbol (&) in php

The reference of php is to add the ampersand in front of the variable, function, object, etc. The reference in PHP means: different names access the same variable content. Here is how to use the reference of PHP. Friends who need it can refer to it. There is a difference between

and pointers in C language. The pointer in C language stores a reference to the address variable where the content of the variable is stored in memory.

PHP’s reference allows you to use two variables to point to the same content

$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

Function call by address

I won’t go into details about the call by address. The code will be given directly below

<?php
function test(&$a) {
  $a = $a + 100;
}
$b = 1;
echo $b; //输出１
test($b); //这里$b传递给函数的其实是$b的变量内容所处的内存地址，通过在函数里改变$a的值　就可以改变$b的值了
echo "<br>";
echo $b; //输出101

?>

It should be noted that if test(1); is used here, an error will occur. Think about the reason yourself.

The reference return of the function

Look at the code first

<?php
function &test() {
  static $b = 0; //申明一个静态变量
  $b = $b + 1;
  echo $b;
  return $b;
}
$a = test(); //这条语句会输出　$b的值　为１
$a = 5;
$a = test(); //这条语句会输出　$b的值　为2
$a = & test(); //这条语句会输出　$b的值　为3
$a = 5;
$a = test(); //这条语句会输出　$b的值　为6

?>

The following explanation:

In this way, $a=test(); is not actually returned by a reference to the function, which is different from ordinary There is no difference in function calls. As for the reason: This is the regulation of PHP.
PHP stipulates that the reference return of the function is obtained through $a=&test();.
As for what is a reference return (the PHP manual says: Reference return is used when you want to use a function to find which variable the reference should be bound to.) This nonsense made me unable to understand it for a long time

Use the above example to explain it

Calling a function with $a=test() only assigns the value of the function to $a, and any changes to $a will not affect $b in the function. Through $a=&test () method calls a function, its function is to point the memory address of the $b variable in return $b and the memory address of the $a variable to the same place, which produces the equivalent of this effect ($a=&b; ) So changing the value of $a also changes the value of $b, so after executing

##$a=&test(); $a=5
;

After that, the value of $b becomes 5

The static variable is used here to let everyone understand the reference return of the function. In fact, the reference return of the function is mostly used in objects. Reference to the

object

<?php
class a {
  var $abc = "ABC";
}
$b = new a;
$c = $b;
echo $b->abc; //这里输出ABC
echo $c->abc; //这里输出ABC
$b->abc = "DEF";
echo $c->abc; //这里输出DEF

?>

The above code is the running effect in PHP5

In PHP5, object copying is achieved through references. In the above column, $b=new a; $c=$b; is actually equivalent to $b=new a; $c=&$b;

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.

quoted Function

If the program is relatively large, there are many variables referencing the same object, and you want to manually clear the object after using it, I personally recommend using the "&" method, and then using $var =null method to clear. Otherwise, use the default method of php5. In addition, for the transfer of large arrays in php5, it is recommended to use the "&" method, after all, it saves memory space.

Unreference

When you unset a reference, you just break the binding between the variable name and the variable content. This does not mean that the variable contents are destroyed. For example:

$a = 1; 
$b =& $a; 
unset ($a);

will not unset $b, just $a.

global reference

When declaring a variable with global $var, a reference to the global variable is actually established. That is the same as doing:

$var =& $GLOBALS["var"];

This means that, for example, unset $var will not unset a global variable.

$this

In a method of an object, $this is always a reference to the object that calls it.

//Here’s another little episode

The pointing (similar to pointer) function of the address in php is not implemented by the user himself, but is implemented by the Zend core , the reference in PHP adopts the principle of "copy-on-write", that is, unless a write operation occurs, variables or objects pointing to the same address will not be copied.

In layman terms

1: If there is the following code

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

In fact, at this time $a and $b both point to the same memory address rather than $a and $b occupying different memories 2: If you add the following code to the above code

$a="EFG";

The above is the entire content of this article, I hope it will be helpful to everyone's study.

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