What are PHP quotes? Introduction to quoting in php (code example)

The content of this article is about what is PHP reference? The introduction (code examples) quoted in php has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

1. What is a reference

In PHP, a reference refers to accessing the same variable content with different names.
The variable names and variable contents in PHP are different, so the same content can have different names.
The closest analogy is the Unix file name and the file itself - the variable name is the directory entry, and the variable content is the file itself. References can be thought of as hard links in a Unix file system.

References in PHP are not like pointers in C: for example you cannot do pointer arithmetic on them. The reference is not an actual memory address, but a symbol table alias.

2. Reference features

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

$a =& $b;

This means $a and $b point to the same variable.

If an array with a reference is copied, its value will not be dereferenced. The same goes for passing array values ​​to functions.

$a = 'a';

$arr1 = [
    'a' => $a,
    'b' => &$a, // $arr1['b'] 与 $a 指向同一个变量
];

// 将 $arr1 传值赋值给 $arr2
$arr2 = $arr1;

print_r($arr2); // $arr2 的值为 ['a' => 'a', 'b' => 'a']

// 修改 $a 的值为 'b'
$a = 'b';

print_r($arr2); // $arr2 的值为 ['a' => 'a', 'b' => 'b']


function foo($arr){
    // 将 $arr['b'] 的值改为 'c';
    $arr['b'] = 'c';
}

echo $a; // $a 的值为 'b'

// 将 $arr1 传入函数
foo($arr1);

echo $a; // $a 的值为 'c'

If an undefined variable is assigned by reference, passed by reference parameter, or returned by reference, the variable will be automatically created.

// 定义函数 foo()，通过引用传递参数
function foo(&$var) { }

foo($a); // 创建变量 $a，值为 NULL
var_dump($a); // NULL

foo($b['b']); // 创建数组 $b = ['b' => NULL]
var_dump(array_key_exists('b', $b)); // bool(true)

$c = new StdClass;
foo($c->d); // 创建对象属性 $c->d = NULL
var_dump(property_exists($c, 'd')); // bool(true)

If a reference is assigned to a variable declared as global inside a function, the reference is only visible inside the function. This can be avoided by using the $GLOBALS array.

$var1 = 'var1';
$var2 = 'var2';

function global_references($use_globals)
{
    global $var1, $var2;
    if (!$use_globals) {
        $var2 = & $var1; // $var2 只在函数内部可见
    } else {
        $GLOBALS["var2"] = & $var1; // $GLOBALS["var2"]在全球范围内也可见
    }
}

global_references(false);
echo "var2 is set to '$var2'\n"; // var2 is set to 'var2'
global_references(true);
echo "var2 is set to '$var2'\n"; // var2 is set to 'var1'

You can think of global $var; as the abbreviation of $var =& $GLOBALS['var'];. Thus assigning another reference to $var only changes the reference to the local variable.

Assign a value to a variable with a reference in the foreach statement, and the referenced object is also changed.

$ref = 0;
$row = & $ref;
foreach ([1, 2, 3] as $row) {
    // do something
}
echo $ref; // 3 - 遍历数组的最后一个元素

3. Pass by reference

You can pass a variable to a function by reference, so that the function can modify the value of its parameter.

function foo(&$var)
{
    $var++;
}

$a=5;
foo($a);

echo $a; // 6

Note that there are no reference symbols in the function call - only in the function definition. The function definition alone is enough for parameters to be passed correctly by reference.

What can be passed by reference:

• Variables

• References returned from functions

Passing variables by reference

function foo(&$var)
{
    $var++;
}

$a=5;
foo($a);

echo $a; // 6

Passing by reference From a function Returned reference

function foo(&$var)
{
    $var++;
    echo $var; // 6
}

function &bar()
{
    $a = 5;
    return $a;
}

foo(bar());

You cannot pass functions, expressions, values, etc. by reference

function foo(&$var)
{
    $var++;
}

function bar() // 注意，这个函数不返回引用
{
    $a = 5;
    return $a;
}

foo(bar()); // 自 PHP 5.0.5 起导致致命错误，自 PHP 5.1.1 起导致严格模式错误，自 PHP 7.0 起导致 notice 信息

foo($a = 5); // 表达式，不是变量。PHP Notice:  Only variables should be passed by reference

foo(5); // PHP Fatal error:  Only variables can be passed by reference

4. Return by reference

Return by reference can be used when you want to use a function to find the variable to which a reference should be bound.
Don't use return references to increase performance, the engine is smart enough to optimize itself. Only return references if there is a valid technical reason!

class Foo {
    public $value = 42;

    public function &getValue() {
        return $this->value;
    }
}

$foo = new Foo;
// $myValue 是 $obj->value 的引用.
$myValue = &$foo->getValue();
// 将 $foo->value 修改为 2
$foo->value = 2;
echo $myValue;  // 2

Reference return can only return variables. If you try to return a reference from a function like this: return intval($this->value);, you will get an error because the function is trying to return the result of an expression rather than a referenced variable. You can only return reference variables from functions - there is no other way.

class Foo {
    public $value = 42;

    public function &getValue() {
        return intval($this->value); // PHP Notice:  Only variable references should be returned by reference
    }
}

$foo = new Foo;
// $myValue 是 $obj->value 的引用.
$myValue = &$foo->getValue();

5. 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.

$a = 1;
$b = & $a;
unset($a);

echo $b; // 1

6. Found that

Many PHP syntax structures are implemented through the reference mechanism, so everything above about reference binding also applies to these structures.

global reference

When you declare a variable with global $var, you actually create a reference to the global variable inside the function. In other words, the effect of doing this is the same:

global $var;

$var =& $GLOBALS["var"];

This means that unset $var will not unset the global variable $GLOBALS["var"].

$this

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

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