PHP variable scope, reference, object reference, transfer_PHP tutorial

This article introduces the variable scope, reference, object reference, and transfer of PHP. Friends in need can refer to it.

Variable scope
The scope of a variable is the context in which it is defined (that is, its effective scope). Most PHP variables have only a single scope. This single scope span also includes files introduced by include and require. For example:

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$a = 1; include 'b.inc'; ?>

$a = 1;

include 'b.inc';

?>

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$a = 1; /* global scope */ function Test() {     echo $a; /* reference to local scope variable */ } Test(); ?>

This variable $a will take effect in the included file b.inc. However, in user-defined functions, a local function scope will be introduced. Any variables used inside a function will be restricted to the local function scope by default. For example:

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$a = 1; /* global scope */

function Test()

{

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

echo $a; /* reference to local scope variable */

}

Test();

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function test(&$a) { $a=$a+100; } $b=1; echo $b;//输出１ echo " "; test($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 " "; echo $b;//输出101 ?>

?>

This script will produce no output because the echo statement refers to a local version of the variable $a, and it is not assigned a value within this scope. You may notice that PHP's global variables are a little different from C language. In C language, global variables automatically take effect in functions unless overridden by local variables. This may cause some problems, someone may accidentally change a global variable. Global variables in PHP must be declared global when used in functions. The meaning of quoting in PHP is: different names access the same variable content. It 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 Variable reference

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

Pass-by-reference call of function I won’t go into details about call by address. The code is given directly below

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function test(&$a) { $a=$a+100; } $b=1; echo $b;//output 1 echo " "; test($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 ?>

It should be noted that if test(1); is used here, an error will occur. Think about the reason yourself
Function reference returns
Let’s look at the code first

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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 echo " "; $a=5; $a=test();//这条语句会输出　$b的值　为2 echo " "; $a=&test();//这条语句会输出　$b的值　为3 echo " "; $a=5; $a=test();//这条语句会输出　$b的值　为6 ?>

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

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class a{ public $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 ?>

$a=5;

$a=test();//This statement will output that the value of $b is 6
?>

Explain below: In this way, $a=test(); actually does not get a function reference return, which is no different from an ordinary function call. As for the reason: this is a PHP regulation.

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class a{ public $abc="ABC"; } $b=new a; $c=$b; $d = clone$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 echo $b->abc;//这里输出DEF $d->abc="111"; echo $d->abc;//这里输出111 echo $b->abc;//这里输出DEF，说明clone后的副本$d对象的改变并没有影响$b对象 ?>

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 nonsense made me not understand it for a long time To explain using the above example, 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 the function. In fact, the reference return of the function is mostly used in objects Object reference

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class a{ public $abc="ABC"; } $b=new a; $c=$b; echo $b->abc;//Output ABC here 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 . For example, the following example

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class a{ public $abc="ABC"; } $b=new a; $c=$b; $d = clone$b; echo $b->abc;//Output ABC here echo $c->abc;//Output ABC here $b->abc="DEF"; echo $c->abc;//Output DEF here echo $b->abc;//Output DEF here $d->abc="111"; echo $d->abc;//The output here is 111 echo $b->abc;//DEF is output here, indicating that changes to the $d object after cloning do not affect the $b object ?>

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 Way. In addition, in php5
For transferring large arrays, it is recommended to use the "&" method, after all, it saves memory space.

Unquote
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:

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$a = 1; $b = &$a; unset ($a); var_dump($a);//这里输出null var_dump($b);//这里输出int 1 ?>

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$a = 1;
$b = &$a;
unset ($a);
var_dump($a);//null is output here
var_dump($b);//Int 1 is output here
?>

Won’t unset $b, just $a.

global quote
When you declare a variable with global $var you actually create a reference to the global variable. That is the same as doing this:
$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", which means that unless a write operation occurs, it points to the same address. The variable or object is

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

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echo "my favorite movie site is :"; echo $_GET['favmovie']; echo ""; $movierate= 5 ; echo "my favorite movie rating for this movie is "; echo  $movierate; ?>

$a="ABC";

$b=$a;

$a="EFG";

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

Learn variable passing in php!

1 Pass variables using url

Example

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//$myfavmovie=urlencode("life of brian");  echo "";  echo "click here to see more information about my favorite movie!";  echo "";  ?>

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echo "my favorite movie site is :"; echo $_GET['favmovie']; echo ""; $movierate= 5 ; echo "my favorite movie rating for this movie is "; echo $movierate; ?> Save this file as movie1.php. Use $_GET['favmovie'] to receive the variables passed by the url! Write to another file and save as moviesite.php

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//$myfavmovie=urlencode("life of brian"); echo ""; echo "click here to see more information about my favorite movie!"; echo ""; ?> http://www.bkjia.com/PHPjc/629033.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/629033.htmlTechArticleThis article introduces the variable scope, reference, object reference, and transfer of PHP. Friends in need can refer to it. Variable scope The scope of a variable is the context in which it is defined (that is, its...