??操作符是PHP 7引入的空合并操作符，用于簡(jiǎn)潔地處理null值檢查。 1. 它首先檢查變量或數(shù)組鍵是否存在且不為null，若是則返回該值，否則返回默認(rèn)值，如$array['key'] ?? 'default'。 2. 相比isset()與三元運(yùn)算符結(jié)合的方式，??更簡(jiǎn)潔且支持鏈?zhǔn)秸{(diào)用，如$_SESSION'user'['theme'] ?? $_COOKIE['theme'] ?? 'light'。 3. 常用于安全處理表單輸入、配置讀取和對(duì)象屬性訪問，但僅判斷null，不識(shí)別''、0或false為“空”。 4. 使用時(shí)需注意避免訪問不存在的嵌套結(jié)構(gòu)導(dǎo)致警告，可通過輔助函數(shù)如array_get實(shí)現(xiàn)安全遍歷。 5. 盡管功能有限，??在提升代碼可讀性和安全性方面效果顯著，是現(xiàn)代PHP開發(fā)中的推薦實(shí)踐。

Null checks are a common part of any PHP application. Whether you're pulling data from a database, processing user input, or reading configuration values, you've probably written more than a few isset() or !empty() checks in your time. But there's a cleaner, more concise way to handle these scenarios — and it starts with two simple question marks: ?? .

What Is the Null Coalescing Operator ( ?? )?

The ?? operator, known as the null coalescing operator , was introduced in PHP 7. It provides a neat way to check whether a variable or array key exists and is not null — and if it isn't, fall back to a default value.

Here's how it works:

 $value = $array['key'] ?? 'default';

This line means:
"If $array['key'] exists and is not null, assign it to $value . Otherwise, use 'default' ."

It's functionally similar to:

 $value = isset($array['key']) ? $array['key'] : 'default';

But much cleaner and easier to read — especially when chaining multiple checks.

Why ?? Beats Ternary isset()

Before PHP 7, handling undefined array keys safely meant wrapping ternary expressions in isset() :

 $username = isset($_GET['user']) ? $_GET['user'] : 'guest';

Not only is this verbose, but nesting these checks quickly becomes messy:

 $theme = isset($_SESSION['user']['preferences']['theme']) 
    ? $_SESSION['user']['preferences']['theme'] 
    : 'light';

With ?? , you can simplify deeply nested fallbacks:

 $theme = $_SESSION['user']['preferences']['theme'] ?? 'light';

And even chain multiple fallbacks:

 $theme = $_SESSION['user']['preferences']['theme'] 
    ?? $_COOKIE['theme'] 
    ?? 'light';

Each operand is evaluated left to right, returning the first one that exists and is not null.

Common Use Cases for ?? in Real Applications

Using ?? effectively can make your code more robust and readable. Here are some practical examples:

• Handling form inputs safely:

   $email = $_POST['email'] ?? null;

• Fetching configuration with fallbacks:

   $apiUrl = config('services.api.url') ?? 'https://api.example.com';

• Working with optional object properties:

   $title = $post->title ?? 'No title provided';

?? Note: ?? only checks for null . It will return values like '' , 0 , or false . If you need to treat those as "empty" too, stick with empty() or combine checks as needed.

Avoiding Common Pitfalls

While ?? is powerful, it's important to understand what it doesn't do:

• It doesn't check if a value is logically empty — just if it's set and not null.
• It won't prevent warnings if you access an invalid array structure without proper nesting checks.

For example:

 // This may still cause a warning if $_SESSION['user'] doesn't exist
$theme = $_SESSION['user']['preferences']['theme'] ?? 'light';

To avoid this, ensure the full path exists or use nested ?? carefully. One workaround is to use functions or helpers that safely traverse arrays:

 function array_get($array, $path, $default = null) {
    foreach (explode('.', $path) as $key) {
        if (!is_array($array) || !isset($array[$key])) {
            return $default;
        }
        $array = $array[$key];
    }
    return $array;
}

$theme = array_get($_SESSION, 'user.preferences.theme', 'light');

But for simple cases, ?? still wins in clarity and speed.

Basically, ?? is a small syntax feature that brings big improvements in code readability and safety when dealing with potentially null values. Once you start using it, you'll wonder how you lived without it.

以上是'??'的功能：簡(jiǎn)化您的PHP應(yīng)用程序中的無效檢查的詳細(xì)內(nèi)容。更多信息請(qǐng)關(guān)注PHP中文網(wǎng)其他相關(guān)文章！