php reference (&) usage

Quoted in PHP means: different names access the same variable content. PHP is different from pointers in C language. The pointer in C language stores the address where the content of the variable is stored in memory. Let’s take a look at php Quote (&) usage

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

<?php
$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 say more about call by address. The code is given directly below.

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); here, an error will occur. Think about the reason yourself
The reference return of the function
Look at the code first

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

Explained below Next:
What you get through $a=test(); in this way is not actually a function reference return, which is no different from an ordinary function call. As for the reason: This is the regulation of PHP
PHP stipulates that $a= The &test(); method obtains the reference return of the function.
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.
Using the above example to explain, it means that calling a function in $a=test() method only assigns the value of the function to $a, and $a does not make any changes. It will not affect $b
in the function. If the function is called through $a=&test(), its function is to point the memory address of the $b variable in return $b to the memory address of the $a variable. The same place
produces the equivalent of this effect ($a=&b;), so changing the value of $a also changes the value of $b, so after executing

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

, the value of $b changes to 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
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, the copying of objects 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 the object 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.
The role of references
If the program is relatively large, references There are many variables in the same object, and you want to clear it manually after using the object. I personally recommend using the "&" method, and then using $var=null to clear it.
OthersIn other cases, use the default of php5 Method. 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:

<?php
$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:

<?php
$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, and is quoted in php It is 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.
Popular speaking
1: If there is the following code

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

In fact, $a and $b point to the same memory address at this time, not that $a and $b occupy different spaces. Memory

2: If you add the following code to the above code

$a="EFG";

because $a and $b point to The data in the memory needs to be rewritten. At this time, the Zend core will automatically determine and automatically produce a data copy of $a for $b, and reapply for a piece of memory for storage.

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