純函數(shù)是函數(shù)式編程中的概念，在PHP中可通過遵循特定規(guī)則實(shí)現(xiàn)，其核心特徵包括：1.無副作用，不修改全局變量、對(duì)象狀態(tài)或執(zhí)行I/O操作；2.相同輸入始終返回相同輸出，不依賴時(shí)間、隨機(jī)數(shù)等外部數(shù)據(jù)；3.保持簡單專注，僅處理輸入並返回結(jié)果，不使用引用修改或靜態(tài)變量保留狀態(tài)。例如，sum(int $a, int $b): int是一個(gè)典型的純函數(shù)，它僅依賴參數(shù)且無外部影響。

A pure function is a concept from functional programming that refers to a function which, given the same input, will always return the same output and has no side effects. This means it doesn't modify any external state or rely on external variables — it only depends on its parameters.

In PHP, while not strictly functional, you can still write pure functions by following some basic rules.

No Side Effects

A pure function shouldn't change anything outside of itself. That includes:

• Not modifying global variables
• Not changing object states (if passed in)
• Not performing I/O operations like writing to a file or making API calls

Example:

 // Impure example – modifies an external variable
$counter = 0;
function increment() {
    global $counter;
    return $counter;
}

 // Pure version – takes value as input, returns new value
function increment($value) {
    return $value 1;
}

The second version is better because it's predictable and easier to test. You know what goes in and exactly what comes out.

Consistent Output for Same Input

A pure function must always return the same result when given the same arguments. It shouldn't rely on randomness, current time, or any hidden data.

Avoid this:

 function getCurrentTime() {
    return time(); // Different every time – impure
}

Instead, if you need time-based logic, pass the time as an argument:

 function formatTime($timestamp) {
    return date('Ymd H:i:s', $timestamp); // Pure
}

This makes your code more testable and less dependent on environment state.

Keep It Simple and Focused

Pure functions tend to be small and focused on one task. They take input, process it, and return output — nothing else.

Here are a few tips:

• Use scalar types ( int , string , bool ) or arrays as inputs and outputs where possible.
• Avoid using &$variables for reference changes.
• Don't use static variables inside the function to retain state.

Example of a pure PHP function:

 function sum(int $a, int $b): int {
    return $a $b;
}

This function is completely self-contained. It doesn't touch anything else, and it gives a reliable result every time.

That's basically how you write a pure function in PHP. It's not hard, but it does require being mindful of dependencies and side effects.

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