In PHP, variables are passed by value by default, meaning functions or assignments receive a copy of the data, while passing by reference allows modifications to affect the original variable. 1. When passing by value, changes to the copy do not impact the original, as shown when assigning $b = $a or passing $x to addOne(). 2. Passing by reference with & makes variables point to the same data, so modifying one affects the other, useful for large arrays or updating input values. 3. Objects in PHP 5 behave like references by default, meaning multiple variables can point to the same object unless explicitly cloned. References should be used carefully to avoid confusion and maintain clean, debuggable code.

In PHP, understanding how variables are passed by value versus by reference helps avoid unexpected behavior in your code. The difference boils down to whether a function or assignment gets its own copy of the data (by value) or works with the original variable (by reference).

Passing Variables by Value

By default, PHP passes variables by value. That means when you assign a variable to another or pass it into a function, a copy of the value is made.

For example:

$a = 10;
$b = $a; // $b gets the value of $a at this moment
$a = 20;

echo $b; // Still prints 10

This also applies inside functions:

function addOne($num) {
    $num  = 1;
}

$x = 5;
addOne($x);
echo $x; // Still 5 because only the copy was changed

So unless told otherwise, PHP won’t change the original variable when passing it around — it uses a copy instead.

Passing Variables by Reference

To make a function or assignment affect the original variable, you need to use references, indicated with the & symbol.

Here’s how you do it:

• In assignments:

  $a = 10;
  $b = &$a; // $b now refers to the same variable as $a
  $a = 20;
  echo $b; // Prints 20

• In function parameters:

  function addOne(&$num) {
    $num  = 1;
  }

$x = 5; addOne($x); echo $x; // Now prints 6

Use cases for references include:
- Modifying large arrays without copying them
- Working with objects before PHP 5 (objects are passed by reference by default since then)
- Callbacks or functions that logically need to update input values

But be careful — using references can make code harder to follow and debug if overused or used inconsistently.

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### When Objects Come Into Play

Starting from PHP 5, objects are a special case. Even when assigned or passed normally, they behave like references. That means:

```php
class Example {
    public $value = 10;
}

$obj1 = new Example();
$obj2 = $obj1;
$obj1->value = 20;

echo $obj2->value; // Prints 20

Even though we didn't use &, both variables point to the same object. If you want $obj2 to be a true copy, you need to explicitly clone it:

$obj2 = clone $obj1;

This is an important detail — while objects act like references by default, they're not exactly the same thing as using &.

Passing by value vs. by reference in PHP affects how changes propagate through your application. Stick to passing by value unless you have a clear reason to use references. And remember, with objects, you're already working with something close to a reference even without the &.基本上就這些.

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