==和===的区别是什么?
松==比较究竟是如何工作的? 严格的===比较到底是如何工作的?
有什么有用的例子吗?
当前回答
<?php
/**
* Comparison of two PHP objects == ===
* Checks for
* 1. References yes yes
* 2. Instances with matching attributes and its values yes no
* 3. Instances with different attributes yes no
**/
// There is no need to worry about comparing visibility of property or
// method, because it will be the same whenever an object instance is
// created, however visibility of an object can be modified during run
// time using ReflectionClass()
// http://php.net/manual/en/reflectionproperty.setaccessible.php
//
class Foo
{
public $foobar = 1;
public function createNewProperty($name, $value)
{
$this->{$name} = $value;
}
}
class Bar
{
}
// 1. Object handles or references
// Is an object a reference to itself or a clone or totally a different object?
//
// == true Name of two objects are same, for example, Foo() and Foo()
// == false Name of two objects are different, for example, Foo() and Bar()
// === true ID of two objects are same, for example, 1 and 1
// === false ID of two objects are different, for example, 1 and 2
echo "1. Object handles or references (both == and ===) <br />";
$bar = new Foo(); // New object Foo() created
$bar2 = new Foo(); // New object Foo() created
$baz = clone $bar; // Object Foo() cloned
$qux = $bar; // Object Foo() referenced
$norf = new Bar(); // New object Bar() created
echo "bar";
var_dump($bar);
echo "baz";
var_dump($baz);
echo "qux";
var_dump($qux);
echo "bar2";
var_dump($bar2);
echo "norf";
var_dump($norf);
// Clone: == true and === false
echo '$bar == $bar2';
var_dump($bar == $bar2); // true
echo '$bar === $bar2';
var_dump($bar === $bar2); // false
echo '$bar == $baz';
var_dump($bar == $baz); // true
echo '$bar === $baz';
var_dump($bar === $baz); // false
// Object reference: == true and === true
echo '$bar == $qux';
var_dump($bar == $qux); // true
echo '$bar === $qux';
var_dump($bar === $qux); // true
// Two different objects: == false and === false
echo '$bar == $norf';
var_dump($bar == $norf); // false
echo '$bar === $norf';
var_dump($bar === $norf); // false
// 2. Instances with matching attributes and its values (only ==).
// What happens when objects (even in cloned object) have same
// attributes but varying values?
// $foobar value is different
echo "2. Instances with matching attributes and its values (only ==) <br />";
$baz->foobar = 2;
echo '$foobar' . " value is different <br />";
echo '$bar->foobar = ' . $bar->foobar . "<br />";
echo '$baz->foobar = ' . $baz->foobar . "<br />";
echo '$bar == $baz';
var_dump($bar == $baz); // false
// $foobar's value is the same again
$baz->foobar = 1;
echo '$foobar' . " value is the same again <br />";
echo '$bar->foobar is ' . $bar->foobar . "<br />";
echo '$baz->foobar is ' . $baz->foobar . "<br />";
echo '$bar == $baz';
var_dump($bar == $baz); // true
// Changing values of properties in $qux object will change the property
// value of $bar and evaluates true always, because $qux = &$bar.
$qux->foobar = 2;
echo '$foobar value of both $qux and $bar is 2, because $qux = &$bar' . "<br />";
echo '$qux->foobar is ' . $qux->foobar . "<br />";
echo '$bar->foobar is ' . $bar->foobar . "<br />";
echo '$bar == $qux';
var_dump($bar == $qux); // true
// 3. Instances with different attributes (only ==)
// What happens when objects have different attributes even though
// one of the attributes has same value?
echo "3. Instances with different attributes (only ==) <br />";
// Dynamically create a property with the name in $name and value
// in $value for baz object
$name = 'newproperty';
$value = null;
$baz->createNewProperty($name, $value);
echo '$baz->newproperty is ' . $baz->{$name};
var_dump($baz);
$baz->foobar = 2;
echo '$foobar' . " value is same again <br />";
echo '$bar->foobar is ' . $bar->foobar . "<br />";
echo '$baz->foobar is ' . $baz->foobar . "<br />";
echo '$bar == $baz';
var_dump($bar == $baz); // false
var_dump($bar);
var_dump($baz);
?>
其他回答
变量有类型和值。
$var = "test"是一个包含"test"的字符串 $var2 = 24是一个整数，其值为24。
当您使用这些变量(在PHP中)时，有时您没有好的类型。 例如，如果你这样做
if ($var == 1) {... do something ...}
PHP必须将$var转换为整数。在本例中，"$var == 1"为真，因为任何非空字符串都被转换为1。
当使用===时，检查值和类型是否相等，因此"$var === 1"为假。
这很有用，例如，当你有一个函数可以返回false(错误)和0(结果):
if(myFunction() == false) { ... error on myFunction ... }
这段代码是错误的，如果myFunction()返回0，它被强制转换为false，你似乎有一个错误。正确的代码是:
if(myFunction() === false) { ... error on myFunction ... }
因为测试的是返回值“是一个布尔值并且为假”，而不是“可以被强制转换为假”。
运算符==在两个不同类型之间进行类型强制转换，而===运算符执行“类型安全比较”。这意味着只有当两个操作数具有相同的类型和相同的值时，它才返回true。
例子:
1 === 1: true
1 == 1: true
1 === "1": false // 1 is an integer, "1" is a string
1 == "1": true // "1" gets casted to an integer, which is 1
"foo" === "foo": true // both operands are strings and have the same value警告:具有等价成员的同一个类的两个实例不匹配===操作符。例子:
$a = new stdClass();
$a->foo = "bar";
$b = clone $a;
var_dump($a === $b); // bool(false)
==(相等)和===(完全相等)之间的差值
PHP提供了两个比较运算符来检查两个值是否相等。这两者之间的主要区别是'=='检查两个操作数的值是否相等。另一方面，'==='检查操作数的值以及操作数的类型是否相等。
= =(等于) ===(完全相等)
示例= >
<?php
$val1 = 1234;
$val2 = "1234";
var_dump($val1 == $val2);// output => bool(true)
//It checks only operands value
?>
<?php
$val1 = 1234;
$val2 = "1234";
var_dump($val1 === $val2);// output => bool(false)
//First it checks type then operands value
?>
如果将$val2转换为(int)$val2或(string)$val1，则返回true
<?php
$val1 = 1234;
$val2 = "1234";
var_dump($val1 === (int)$val2);// output => bool(true)
//First it checks type then operands value
?>
OR
<?php
$val1 = 1234;
$val2 = "1234";
var_dump($val1 === (int)$val2);// output => bool(true)
//First it checks type then operands value
?>
<?php
/**
* Comparison of two PHP objects == ===
* Checks for
* 1. References yes yes
* 2. Instances with matching attributes and its values yes no
* 3. Instances with different attributes yes no
**/
// There is no need to worry about comparing visibility of property or
// method, because it will be the same whenever an object instance is
// created, however visibility of an object can be modified during run
// time using ReflectionClass()
// http://php.net/manual/en/reflectionproperty.setaccessible.php
//
class Foo
{
public $foobar = 1;
public function createNewProperty($name, $value)
{
$this->{$name} = $value;
}
}
class Bar
{
}
// 1. Object handles or references
// Is an object a reference to itself or a clone or totally a different object?
//
// == true Name of two objects are same, for example, Foo() and Foo()
// == false Name of two objects are different, for example, Foo() and Bar()
// === true ID of two objects are same, for example, 1 and 1
// === false ID of two objects are different, for example, 1 and 2
echo "1. Object handles or references (both == and ===) <br />";
$bar = new Foo(); // New object Foo() created
$bar2 = new Foo(); // New object Foo() created
$baz = clone $bar; // Object Foo() cloned
$qux = $bar; // Object Foo() referenced
$norf = new Bar(); // New object Bar() created
echo "bar";
var_dump($bar);
echo "baz";
var_dump($baz);
echo "qux";
var_dump($qux);
echo "bar2";
var_dump($bar2);
echo "norf";
var_dump($norf);
// Clone: == true and === false
echo '$bar == $bar2';
var_dump($bar == $bar2); // true
echo '$bar === $bar2';
var_dump($bar === $bar2); // false
echo '$bar == $baz';
var_dump($bar == $baz); // true
echo '$bar === $baz';
var_dump($bar === $baz); // false
// Object reference: == true and === true
echo '$bar == $qux';
var_dump($bar == $qux); // true
echo '$bar === $qux';
var_dump($bar === $qux); // true
// Two different objects: == false and === false
echo '$bar == $norf';
var_dump($bar == $norf); // false
echo '$bar === $norf';
var_dump($bar === $norf); // false
// 2. Instances with matching attributes and its values (only ==).
// What happens when objects (even in cloned object) have same
// attributes but varying values?
// $foobar value is different
echo "2. Instances with matching attributes and its values (only ==) <br />";
$baz->foobar = 2;
echo '$foobar' . " value is different <br />";
echo '$bar->foobar = ' . $bar->foobar . "<br />";
echo '$baz->foobar = ' . $baz->foobar . "<br />";
echo '$bar == $baz';
var_dump($bar == $baz); // false
// $foobar's value is the same again
$baz->foobar = 1;
echo '$foobar' . " value is the same again <br />";
echo '$bar->foobar is ' . $bar->foobar . "<br />";
echo '$baz->foobar is ' . $baz->foobar . "<br />";
echo '$bar == $baz';
var_dump($bar == $baz); // true
// Changing values of properties in $qux object will change the property
// value of $bar and evaluates true always, because $qux = &$bar.
$qux->foobar = 2;
echo '$foobar value of both $qux and $bar is 2, because $qux = &$bar' . "<br />";
echo '$qux->foobar is ' . $qux->foobar . "<br />";
echo '$bar->foobar is ' . $bar->foobar . "<br />";
echo '$bar == $qux';
var_dump($bar == $qux); // true
// 3. Instances with different attributes (only ==)
// What happens when objects have different attributes even though
// one of the attributes has same value?
echo "3. Instances with different attributes (only ==) <br />";
// Dynamically create a property with the name in $name and value
// in $value for baz object
$name = 'newproperty';
$value = null;
$baz->createNewProperty($name, $value);
echo '$baz->newproperty is ' . $baz->{$name};
var_dump($baz);
$baz->foobar = 2;
echo '$foobar' . " value is same again <br />";
echo '$bar->foobar is ' . $bar->foobar . "<br />";
echo '$baz->foobar is ' . $baz->foobar . "<br />";
echo '$bar == $baz';
var_dump($bar == $baz); // false
var_dump($bar);
var_dump($baz);
?>
这都是关于数据类型的。以BOOL (true或false)为例:
True也等于1和 False也等于0
==在比较时不关心数据类型: 所以如果你有一个变量是1(这也可能是真的):
$ var = 1;
然后与==进行比较:
if ($var == true)
{
echo"var is true";
}
但$var并不等于true，对吧?它的int值为1，而int值为true。
使用===，检查数据类型，以确保两个变量/对象/无论使用相同的类型。
如果我这样做了
if ($var === true)
{
echo "var is true";
}
这个条件不会为真，因为$var !== true它只== true(如果你知道我的意思)。
你为什么需要这个?
简单-让我们来看看PHP的一个函数:array_search():
array_search()函数的作用是:简单地在数组中搜索一个值，并返回该值所在元素的键。如果在数组中找不到该值，则返回false。但是，如果对存储在数组的第一个元素(数组键值为0)....中的值执行array_search()会怎样呢array_search()函数将返回0…等于false..
所以如果你有:
$arr = array("name");
if (array_search("name", $arr) == false)
{
// This would return 0 (the key of the element the val was found
// in), but because we're using ==, we'll think the function
// actually returned false...when it didn't.
}
所以，你现在明白这为什么会成为一个问题了吗?
大多数人在检查函数是否返回false时不使用== false。相反，他们使用!但实际上，这与使用==false完全相同，所以如果你这样做:
$arr = array("name");
if (!array_search("name", $arr)) // This is the same as doing (array_search("name", $arr) == false)
因此，对于这种情况，您可以使用===来代替，以便检查数据类型。
推荐文章
- 如何从一个查询插入多行使用雄辩/流利
- 在PHP单元测试执行期间，如何在CLI中输出?
- 在PHP中使用heredoc的优势是什么?
- PHP中的echo, print和print_r有什么区别?
- 如何将XML转换成PHP数组?
- 如何将对象转换为数组?
- 从IP地址获取位置
- 获取数组值的键名
- HTTPS和SSL3_GET_SERVER_CERTIFICATE:证书验证失败，CA is OK
- PHP -获取bool值，当为false时返回false
- 在foreach中通过引用传递
- 如何触发命令行PHP脚本的XDebug分析器?
- 如何找出如果你使用HTTPS没有$_SERVER['HTTPS']
- 更好的方法检查变量为null或空字符串?
- 当使用Composer的开发/生产开关时，如何正确部署?
