PHP中函數(shù)表達(dá)式和函數(shù)聲明的主要區(qū)別在於提升行為和使用場(chǎng)景。函數(shù)聲明以function關(guān)鍵字開(kāi)頭，會(huì)被提升，可在定義前調(diào)用；適用於腳本任意位置可用、無(wú)需條件定義及需要自上而下可讀性的場(chǎng)景。函數(shù)表達(dá)式將函數(shù)賦值給變量，不會(huì)被提升，必須先定義後調(diào)用；適用於需條件創(chuàng)建、使用閉包或匿名函數(shù)及作為參數(shù)傳遞的場(chǎng)景。兩者在功能特性上相同，但創(chuàng)建和訪問(wèn)時(shí)機(jī)不同。

In PHP, functions are a core part of writing reusable and organized code, but how you define them can make a difference in behavior and usage. The main distinction between a function expression and a function declaration lies in how they're written and when they're available for use.

Function Declaration

A function declaration is the more traditional way to define a function. It starts with the function keyword followed by the function name, parameters, and body.

Example:

 function sayHello() {
    echo "Hello!";
}

This kind of definition is hoisted , meaning you can call the function before it's defined in the code. PHP parses all function declarations first, so they're available throughout the scope, even if called earlier in the file.

Use function declarations when:

• You want the function to be available anywhere in your script.
• You don't need to conditionally define the function.
• Your code structure benefits from top-down readability.

Function Expression

A function expression involves assigning a function to a variable. It's often used when passing functions as arguments or returning them from other functions.

Example:

 $greet = function() {
    echo "Hi there!";
};

Here, the function is not hoisted . You must define it before calling it. The function is tied to the variable $greet , and its availability depends on where it appears in the execution flow.

Function expressions are useful when:

• You want to create functions conditionally.
• You're working with closures or anonymous functions.
• You need to pass a function as a value (eg, callbacks).

Key Differences

Let's break down the most important contrasts:

• Hoisting: Function declarations are hoisted; function expressions are not.
• Assignment: Expressions assign the function to a variable, while declarations stand alone.
• Usage context: Declarations are better for general use, expressions offer flexibility in dynamic or callback-based scenarios.

Also worth noting is that both types support the same features like variable scope, parameter passing, and return values. The difference is mainly in how and when they're created and accessed.

That's the basic breakdown. Depending on your needs—whether you're building a simple script or organizing complex logic—you might lean toward one style over the other.

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