What is the relationship between parameter passing methods of PHP functions and reference variables?

In PHP, the method of passing parameters to a function determines how the function accesses variables. Passing by value creates a copy of the variable, while passing by reference operates directly on the original variable. PHP 8 introduced reference type hints, allowing parameters passed by reference to be explicitly specified. Pass by value is suitable for functions that do not modify the original value, while pass by reference is suitable for functions that need to modify the original value.

The relationship between the parameter passing method of the PHP function and the reference variable

In PHP, the parameter passing method of the function determines Describes how the function accesses the variables passed to it. There are two main ways to pass parameters: Pass by value and Pass by reference.

Pass by value

When a variable is passed to a function by value, a copy of the variable is created and passed to the function. This means that the function operates on a copy of the variable, and any changes made to the copy will not affect the original variable.

function add_one($num) {
  $num++; // 对副本进行更改
}

$x = 10;
add_one($x); // 传递变量副本
echo $x; // 输出 10（原始变量保持不变）

Pass by reference

When a variable is passed to a function by reference, the function operates directly on the original variable. Any changes made to the variables in the function are reflected in the original variables.

function add_one(&$num) {
  $num++; // 对原始变量进行更改
}

$x = 10;
add_one($x); // 传递变量引用
echo $x; // 输出 11（原始变量被修改了）

In the above example, the add_one() function passes its argument $num by reference. When the function increments $num, it directly modifies the original variable $x.

Type hints for reference variables

PHP 8 introduced reference type hints, allowing function parameters to be passed by reference to be explicitly specified. Using the & prefix indicates that the parameter should be passed by reference.

function add_one(int &$num) {
  $num++; 
}

Now, when passing $num to the add_one() function, it must be passed by reference. If you don't pass by reference, you'll get an error.

Practical cases

The following are practical cases for passing by value and passing by reference:

Pass by value:

• Array sort or trim functions, such as sort() or trim(), will not Modify the original array or string.

Pass by reference:

• Functions that update database records or file system contents require direct manipulation of the original variables.

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