Understanding and using PHP's & quote character_PHP Tutorial-PHP Tutorial-php.cn

Understanding and using PHP's & quote character_PHP Tutorial

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Release： 2016-07-13 10:33:29

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Regarding the role of PHP references (that is, adding the ampersand in front of variables, functions, objects, etc.), let’s first look at the following program.

    
<?php
$a = 100; //声明变量a
$b = &$a; //声明变量b,引用自变量a
echo "$a <br />";  
echo "$b <br />";
$a++; //变量a自增1
echo "$a <br />";
echo "$b <br />";//查看变量b,也增加了1,说明使用的是同一存储单元
?>

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Program execution result:

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Many people misunderstand that references in PHP are the same as pointers in C. In fact, they are not, and they are very different. Except for the pointers in C language that do not need to be explicitly declared during the array transfer process, other points need to be defined using *. However, the pointer to address (similar to a pointer) function in PHP is not implemented by the user himself, but is implemented by the Zend core. Yes, 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.

php defaults to passing by value:

 
<?php    
$a = 20;
$b = $a;
$a = $a + 10; 
echo $a.' and '.$b; 
?>

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Program execution result:

30 and 20

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If you want to pass it as an address, you need to add &, that is:

  
<?php
$a = 20;
$b = &$a; 
$a = $a + 10; 
echo $a.' and '.$b; 
?>

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Program execution result:

30 and 30

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In other words, & passes the address of $a to $b. In this case, the two variables now share a memory storage area, which means that their values are the same.

The same syntax can be used in functions, which return references, and in the new operator:

<?php
$bar =& new fooclass();
$foo =& find_var($bar);
?>

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The second thing a reference does is pass a variable by reference. This is accomplished by creating a local variable within the function that references the same content in the calling scope. To put it simply, the parameter of a function is a reference to a local variable. Here’s another example:

<?php
function foo(&$val1, $val2) {
	$val1 += 1;
	$val2 += 1;
}
$a=5;
$b=10;
foo($a,$b);
echo $a;
echo $b;
?>

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To run this code, you pass two parameters to the function, one is a reference to the content of $a, and the other is the value of $b. After executing this function, it is found that the content of $a has changed, while the content of $b is No changes.

The third usage of PHP references is reference return. This usage is a bit difficult to understand. Reference return is used when you want to use a function to find which variable the reference should be bound to. When returning a reference, use this syntax: To put it simply, it is still the return of the reference function. But unlike parameter passing, the & symbol must be used in both function definition and function reference. Here’s an example:

<?php
function &find_var ($param)
{
    /* ...code... */
    return $found_var;
}
$foo =& find_var ($bar);
$foo->x = 2;
?>

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In this example, the assignment to $foo is the return reference of the function find_var, so when assigning a value to $foo->x, the return reference of find_var is assigned instead of a simple assignment.

The last usage of PHP references is reference positioning. There are two main applications: one is global reference. When declaring a variable with global $var, a reference to the global variable is actually established. That is, it is the same as $var =& $GLOBALS["var"];. Another one is the usage of $this. In a method of an object, $this is always a reference to the object that calls it.