短路求值是PHP中邏輯運算符的重要特性，能提升性能並避免錯誤。 1. 使用&&時，若左操作數(shù)為假，則不再評估右操作數(shù)；2. 使用||時，若左操作數(shù)為真，則跳過右操作數(shù)；3. 可用於安全調(diào)用對象方法，如if ($user && $user->hasPermission('edit'))避免空對象調(diào)用；4. 能優(yōu)化性能，如跳過昂貴的函數(shù)調(diào)用；5. 可提供默認值，但需注意||對falsy值敏感，可改用??運算符；6. 避免將有副作用的代碼放在可能被跳過的右側(cè)，確保關(guān)鍵操作不被短路。正確使用短路求值可使代碼更安全、簡潔且高效。

Short-circuit evaluation is a powerful yet often underappreciated feature in PHP's logical operators. It allows your code to skip unnecessary evaluations, improving both performance and safety—especially when dealing with conditions that involve function calls, database checks, or null values.

What Is Short-Circuit Evaluation?

In PHP, the logical operators && (AND) and || (OR) use short-circuit evaluation. This means:

• With && , if the left operand is false , the right operand is not evaluated —because the whole expression can't be true .
• With || , if the left operand is true , the right operand is not evaluated —because the expression is already true .

This behavior prevents unnecessary computation and, more importantly, avoids potentially dangerous operations.

Avoiding Errors with Null Checks

One of the most practical uses is guarding against undefined variables or null objects:

 if ($user && $user->hasPermission('edit')) {
    // Edit logic
}

Here, if $user is null or false , PHP won't call hasPermission() , preventing a fatal error. Without short-circuiting, you'd need nested if statements:

 if ($user) {
    if ($user->hasPermission('edit')) {
        // ...
    }
}

Short-circuiting keeps your code flat and readable.

Optimizing Performance

When you chain expensive operations, short-circuiting can save resources:

 if (isUserActive($id) && validateAccessKey($key) && checkRateLimit($id)) {
    // Proceed
}

If isUserActive($id) returns false , the other two functions won't run. This is especially helpful if those functions hit databases or APIs.

Using || for Fallbacks and Lazy Initialization

The OR operator is great for defaults:

 $displayName = $user->getName() || 'Guest';

But be cautious: if getName() returns an empty string or 0 , it will still fall through. For stricter control, consider ?? (null coalescing), which only checks for null :

 $displayName = $user->getName() ?? 'Guest';

Still, || works well when you want any "falsy" value to trigger the fallback.

Watch Out for Side Effects

Short-circuiting can bite you if the skipped expression has side effects:

 if ($valid && logAttempt($result)) {
    // logAttempt() might not run if $valid is false
}

If logging is critical, don't rely on it being called. Move it outside the condition:

 logAttempt($result);
if ($valid) {
    // ...
}

Summary

Short-circuit evaluation in PHP isn't just an optimization—it's a tool for writing safer, cleaner code. Use it to:

• Prevent method calls on null objects
• Skip expensive operations when unnecessary
• Provide fallback behavior
• Keep logic concise

Just remember: if an expression must run, don't bury it in a short-circuited condition.

Basically, work with the flow of logic, not against it.

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