Introduction to passing parameters by reference in PHP: How to use the & symbol

PHP is a popular programming language that supports two ways of passing parameters: passing by value and passing by reference. Passing by value copies the value of one variable to another variable, while passing by reference copies the memory address of one variable to another variable. This article will introduce the use of PHP reference parameters, especially the use of the & symbol.

The basis of passing parameters by reference

In PHP, we can change or return the value of a variable by passing it to a function. This process can be done by passing by value or passing by reference. In pass-by-value, the function copies the value of the variable, whereas in pass-by-reference, the function uses the original variable.

The basic syntax for passing parameters by reference is as follows:

function &example(&$parameter){

// code to modify $parameter
return $parameter;

}

In this function, & represents a reference Passed, $parameter is the parameter of the function. This function will return a reference to the $parameter variable. As you can see, in the function definition, there is an & symbol before the parameter name, which means that this parameter is a reference parameter.

Examples of using the & symbol

Let’s look at some examples to illustrate the use of the & symbol.

// 值传递的例子
function foo($var){
    $var = 2;
} 

$a = 1;
foo($a);
echo $a; // 输出：1

// 引用传递的例子
function bar(&$var){
    $var = 2;
} 

$a = 1;
bar($a);
echo $a; // 输出：2

In this example, the foo() function is a value-passing function, while the bar() function is a reference-passing function. When foo($a) is executed, the value of $a is copied to $var, so the value of $var is 1. Inside the function, the value of $var is set to 2, but since $var is a local variable, its change will not affect the original value of $a.

When bar($a) is executed, the memory address of $a is copied to $var. Therefore, inside the function, the variable $var refers to the memory address of $a, rather than copying the value of $a. When the value of $var is set to 2 inside the function, the value of the $a variable is also changed to 2.

As you can see, by using the & symbol, we can pass variables as pointers to functions. This enables the function to change the value of the original variable, thereby passing mutable state around the program.

It should be noted that the & symbol can only be applied to variables, not expressions. For example, the following code will have a syntax error:

$var = 1;
// 以下代码将会出现语法错误
function foo(1 &$var){ 
    $var = 2;
}

Summary

This article has introduced the basic knowledge of PHP reference passing parameters, especially the use of the & symbol. Passing parameters by reference allows you to change the value of a variable inside a function, which is useful when you need to pass mutable state within a program. It should be noted that the & symbol can only be applied to variables, not expressions. Hope this article can be helpful to readers.

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