Detailed introduction to php reference passing by value

$a="ABC";
$b =&$a;
echo $a;//Output here: ABC
echo $b;//Output here: ABC
$b= "EFG";
echo $a;//The value of $a here becomes EFG, so EFG is output
echo $b;//EFG is output here
?>

2. Transfer address of function transfer

function test(&$a)
{
$a=$a+100;
}
$b=1;
echo $b;//output 1
test($ b); //What $b is passed to the function here is actually the memory address where the variable content of $b is located. By changing the value of $a in the function, the value of $b can be changed
echo "" ;
echo $b;//Output 101

Note that if you test(1); here, an error will occur. You can think about the reason yourself

3. Function reference return

function &test()
{
static $b=0;//Declare a static variable
$b=$b+1;
echo $b;
 return $b;
}
$a=test();//This statement will output the value of $b as 1
$a=5;
$a=test(); //This statement will output the value of $b as 2
$a=&test();//This statement will output the value of $b as 3
$a=5;
$ a=test();//This statement will output the value of $b as 6
?>

Explanation: In this way, $a=test(); actually does not get a reference return from the function. It is no different from an ordinary function call. As for the reason: This is the regulation of PHP. PHP stipulates that what is obtained through $a=&test(); is 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 bullshit made me not understand it for a long time

Using the above example to explain Calling a function using $a=test() only assigns the value of the function to $a, and any changes to $a will not affect $b in the function. When calling a function through $a=&test(), its function is to point the memory address of the $b variable in return $b and the memory address of the $a variable to the same place. That is to say, the effect equivalent to this is produced ($a=&b;), so changing the value of $a also changes the value of $b, so after executing $a=&test(); $a=5; From now on, the value of $b becomes 5

Static variables are used here to let everyone understand the reference return of functions. In fact, the reference return of functions is mostly used in objects

Reference of object:

class a{
var $abc="ABC";
}
$b=new a;
$c=$b;
echo $b->abc;//here Output ABC
echo $c->abc;//Output ABC here
$b->abc="DEF";
echo $c->abc;//Output DEF here
?>

The above code is the effect of running 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 the 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. The role of quotation If the program is relatively large, there are many variables referencing 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. Otherwise, use the default of php5 Method. In addition, for transferring large arrays in php5, it is recommended to use the "&" method, after all, it saves memory space.

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

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

will not unset $b, just $a.

6. global reference When you declare a variable with global $var you actually create a reference to the global variable. That is the same as doing:

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

This means that, for example, unset $var will not unset a global variable. $this In an object method, $this is always a reference to the object that calls it. //Another little interlude below The address pointing (similar to a pointer) function 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, it points to the same address. Variables or objects will not be copied.

In layman’s 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";

Since the data in the memory pointed to by $a and $b needs to be rewritten, the Zend core will automatically judge at this time, automatically generate a data copy of $a for $b, and re-apply for a piece of memory for storage.

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