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    基础

    PHP 中的变量用一个美元符号后面跟变量名来表示。变量名是区分大小写的。

    变量名与 PHP 中其它的标签一样遵循相同的规则。一个有效的变量名由字母或者下划线开头，后面跟上任意数量的字母，数字，或者下划线。 按照正常的正则表达式，它将被表述为：'^[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*$'。

    注意: 在此所说的字母是 a-z，A-Z，以及 ASCII 字符从 128 到 255（0x80-0xff）。

    注意: $this 是一个特殊的变量，它不能被赋值。 PHP 7.1.0 之前，间接赋值（例如通过使用可变变量）是可能的。

    小技巧

    请参见用户空间命名指南。

    有关变量的函数信息见变量函数。

    <?php
    $var     = 'Bob';
    $Var = 'Joe';
    echo     "$var, $Var"; // 输出 "Bob, Joe"

    $4site = 'not yet'; // 非法变量名；以数字开头
    $_4site = 'not yet'; // 合法变量名；以下划线开头
    $i站点is = 'mansikka'; // 合法变量名；可以用中文
    ?>

    变量默认总是传值赋值。那也就是说，当将一个表达式的值赋予一个变量时，整个原始表达式的值被赋值到目标变量。这意味着，例如，当一个变量的值赋予另外一个变量时，改变其中一个变量的值，将不会影响到另外一个变量。有关这种类型的赋值操作，请参阅表达式一章。

    PHP 也提供了另外一种方式给变量赋值：引用赋值。这意味着新的变量简单的引用（换言之，“成为其别名” 或者 “指向”）了原始变量。改动新的变量将影响到原始变量，反之亦然。

    使用引用赋值，简单地将一个 & 符号加到将要赋值的变量前（源变量）。例如，下列代码片断将输出“My name is Bob”两次：

    <?php
    $foo     = 'Bob'; // 将 'Bob' 赋给 $foo
    $bar = &$foo; // 通过 $bar 引用 $foo
    $bar = "My name is $bar"; // 修改 $bar 变量
    echo $bar;
    echo     $foo; // $foo 的值也被修改
    ?>

    有一点重要事项必须指出，那就是只有有名字的变量才可以引用赋值。

    <?php
    $foo     = 25;
    $bar = &$foo; // 合法的赋值
    $bar = &(24 * 7); // 非法; 引用没有名字的表达式

    function test()
    {
    return     25;
    }

    $bar = &test(); // 非法
    ?>

    虽然在 PHP 中并不需要初始化变量，但对变量进行初始化是个好习惯。未初始化的变量具有其类型的默认值 - 布尔类型的变量默认值是 false，整形和浮点型变量默认值是零，字符串型变量（例如用于 echo 中）默认值是空字符串以及数组变量的默认值是空数组。

    示例 #1 未初始化变量的默认值

    <?php
    // 未设置和未引用（不使用上下文）的变量；输出 NULL
    var_dump($unset_var);

    // Boolean 用法；输出 'false' (See ternary operators for more on this syntax)
    echo $unset_bool ? "true\n" : "false\n";

    // String 用法；输出 'string(3) "abc"'
    $unset_str .= 'abc';
    var_dump($unset_str);

    // Integer 用法；输出 'int(25)'
    $unset_int += 25; // 0 + 25 => 25
    var_dump($unset_int);

    // Float 用法；输出 'float(1.25)'
    $unset_float += 1.25;
    var_dump($unset_float);

    // Array 用法；输出 array(1) { [3]=> string(3) "def" }
    $unset_arr[3] = "def"; // array() + array(3 => "def") => array(3 => "def")
    var_dump($unset_arr);

    // Object 用法：创建新 stdClass 对象 (see http://www.php.net/manual/en/reserved.classes.php)
    // Outputs: object(stdClass)#1 (1) { ["foo"]=> string(3) "bar" }
    $unset_obj->foo = 'bar';
    var_dump($unset_obj);
    ?>

    依赖未初始化变量的默认值在某些情况下会有问题，例如把一个文件包含到另一个之中时碰上相同的变量名。 使用未初始化的变量会发出 E_WARNING （在 PHP 8.0.0 之前是 E_NOTICE） 错误，但是在向一个未初始化的数组附加单元时不会。isset() 语言结构可以用来检测一个变量是否已被初始化。

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    User Contributed Notes 2 notes

    up
    75
    jeff dot phpnet at tanasity dot com
    14 years ago
    This page should include a note on variable lifecycle:

    Before a variable is used, it has no existence. It is unset. It is possible to check if a variable doesn't exist by using isset(). This returns true provided the variable exists and isn't set to null. With the exception of null, the value a variable holds plays no part in determining whether a variable is set.

    Setting an existing variable to null is a way of unsetting a variable. Another way is variables may be destroyed by using the unset() construct.

    <?php
    print isset($a); // $a is not set. Prints false. (Or more accurately prints ''.)
    $b = 0; // isset($b) returns true (or more accurately '1')
    $c = array(); // isset($c) returns true
    $b = null; // Now isset($b) returns false;
    unset($c); // Now isset($c) returns false;
    ?>

    is_null() is an equivalent test to checking that isset() is false.

    The first time that a variable is used in a scope, it's automatically created. After this isset is true. At the point at which it is created it also receives a type according to the context.

    <?php
    $a_bool     = true;   // a boolean
    $a_str = 'foo';    // a string
    ?>

    If it is used without having been given a value then it is uninitalized and it receives the default value for the type. The default values are the _empty_ values. E.g  Booleans default to FALSE, integers and floats default to zero, strings to the empty string '', arrays to the empty array.

    A variable can be tested for emptiness using empty();

    <?php
    $a     = 0; //This isset, but is empty
    ?>

    Unset variables are also empty.

    <?php
    empty($vessel); // returns true. Also $vessel is unset.
    ?>

    Everything above applies to array elements too.

    <?php
    $item     = array();
    //Now isset($item) returns true. But isset($item['unicorn']) is false.
    //empty($item) is true, and so is empty($item['unicorn']

    $item['unicorn'] = '';
    //Now isset($item['unicorn']) is true. And empty($item) is false.
    //But empty($item['unicorn']) is still true;

    $item['unicorn'] = 'Pink unicorn';
    //isset($item['unicorn']) is still true. And empty($item) is still false.
    //But now empty($item['unicorn']) is false;
    ?>

    For arrays, this is important because accessing a non-existent array item can trigger errors; you may want to test arrays and array items for existence with isset before using them.
    up
    18
    anisgazig at gmail dot com
    3 years ago
    clear concept of variable declaration rules and classification

    variable declaration rules:

    1.start with dollar sign($)
    2.first letter of variable name comes from a-zA-z_
    3.next letters of variable name comes from a-zA-Z0-9_
    4.no space,no syntex

    classification of variables:

    Variable are mainly Two types
    1.Predefined Variable
    2.User Define Variable

    Predefined Variable
    There are 12 predefined variables in php 8
    1.$GLOBALS
    2.$_SERVER
    3.$_REQUEST
    4.$_FILES
    5.$_ENV
    6.$_SESSION
    7.$_COOKIE
    8.$_GET
    9.$_POST
    10.$http_response_header
    11.$argc
    12.$argv

    User Define Variable
    User Define variable are 3 types
    1.variable scope
    2.variable variables
    3.reference variable

    Variable Scope
    variable scope are 3 types
    1.local scope
    2.global scope
    3.static variable
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