关于JavaScript:

===操作符的工作原理与==操作符相同，但它要求其操作数不仅具有相同的值，而且具有相同的数据类型。

例如，下面的示例将显示'x和y是相等的'，而不是'x和y是相同的'。

var x = 4;
var y = '4';
if (x == y) {
    alert('x and y are equal');
}
if (x === y) {
    alert('x and y are identical');
}

一图胜千言万语:

PHP双等号==等号图表:

PHP三重等号===等号图表:

创建这些图像的源代码:

https://github.com/sentientmachine/php_equality_charts

大师冥想

那些希望保持理智的人，不要再读下去了，因为这些都没有任何意义，只能说PHP的疯狂分形就是这样设计的。

NAN != NAN but NAN == true. == will convert left and right operands to numbers if left is a number. So 123 == "123foo", but "123" != "123foo" A hex string in quotes is occasionally a float, and will be surprise cast to float against your will, causing a runtime error. == is not transitive because "0"== 0, and 0 == "" but "0" != "" PHP Variables that have not been declared yet are false, even though PHP has a way to represent undefined variables, that feature is disabled with ==. "6" == " 6", "4.2" == "4.20", and "133" == "0133" but 133 != 0133. But "0x10" == "16" and "1e3" == "1000" exposing that surprise string conversion to octal will occur both without your instruction or consent, causing a runtime error. False == 0, "", [] and "0". If you add 1 to number and they are already holding their maximum value, they do not wrap around, instead they are cast to infinity. A fresh class is == to 1. False is the most dangerous value because False is == to most of the other variables, mostly defeating it's purpose.

希望:

如果你正在使用PHP，你不应该使用双等号操作符，因为如果你使用三等号，唯一需要担心的边缘情况是NAN和太接近其数据类型最大值的数字，它们会被强制转换为无穷大。使用双等号，任何东西都可以是surprise ==对任何东西或，或可以是出乎你的意愿和!=对显然应该是相等的东西。

在PHP中使用==的任何地方都是糟糕的代码气味，因为隐式强制转换规则暴露了85个错误，这些规则似乎是由数百万程序员通过布朗运动编程设计的。

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)

关于JavaScript:

===操作符的工作原理与==操作符相同，但它要求其操作数不仅具有相同的值，而且具有相同的数据类型。

例如，下面的示例将显示'x和y是相等的'，而不是'x和y是相同的'。

var x = 4;
var y = '4';
if (x == y) {
    alert('x and y are equal');
}
if (x === y) {
    alert('x and y are identical');
}

在PHP数组和对象中，==和===之间有两个没有人提到的区别:两个具有不同键排序的数组和对象。

两个具有不同键排序的数组

如果你有两个数组，它们的键排序不同，但有相同的键值映射，它们是严格不同的(即使用===)。这可能会导致问题，如果你对一个数组进行键排序，并试图将排序后的数组与原始数组进行比较。

例如:

$arrayUnsorted = [
    "you" => "you",
    "I" => "we",
];

$arraySorted = $arrayUnsorted;
ksort($arraySorted);

$arrayUnsorted == $arraySorted; // true
$arrayUnsorted === $arraySorted; // false

对象

请记住，主要规则是两个不同的对象永远不会严格相等。请看下面的例子:

$stdClass1 = new stdClass();
$stdClass2 = new stdClass();
$clonedStdClass1 = clone $stdClass1;

$stdClass1 == $stdClass2; // true
$stdClass1 === $stdClass2; // false
$stdClass1 == $clonedStdClass1; // true
$stdClass1 === $clonedStdClass1; // false

注意:将一个对象赋值给另一个变量并不会创建一个副本——相反，它创建了对同一对象的引用。在这里看到的。

注意:从PHP7开始，引入了匿名类。在上面的测试中，新类{}和新stdClass()之间没有区别。

<?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);
?>