What does value passing mean in php

In PHP, value passing means copying the actual parameters when calling a function and then passing them to the formal parameters of the function. In fact, the formal parameters and actual parameters occupy different storage units respectively. The characteristic of value transfer is one-way transfer, that is, when the calling function is called, a storage unit is allocated to the formal parameter, and the value of the actual parameter is passed to the formal parameter. After the call is completed, the storage unit of the formal parameter is released, and the value of the formal parameter is Any changes will not affect the value of the actual parameter, and the storage unit of the actual parameter still retains and maintains its value unchanged.

The operating environment of this tutorial: windows7 system, PHP8 version, DELL G3 computer

When calling a function, parameters need to be passed to the function. The parameters passed into the function are called actual parameters, while the parameters defined by the function are called formal parameters. There are four ways to pass parameters to a function, namely passing by value, passing by reference, default parameters and variable length parameters.

Value passing

Value passing is the default value passing method for functions in PHP, also known as "copy passing by value". As the name suggests, the value passing method will copy the value of the actual parameter and then pass it to the formal parameter of the function, so operating the value of the parameter in the function will not affect the actual parameters outside the function. Therefore, if you do not want the function to modify the value of the actual parameter, you can pass it by value.

The characteristic of value transfer is one-way transfer, that is, when the calling function is called, a storage unit is allocated to the formal parameter, and the value of the actual parameter is transferred to the formal parameter. After the call is completed, the formal parameter is stored The unit is released, and any change in the formal parameter value will not affect the value of the actual parameter. The storage unit of the actual parameter still retains and maintains the value unchanged.

[Example] Define a simple function below. The function has two parameters, and the values ​​of the parameters are exchanged in the function.

<?php
    function swap($a, $b){
        echo &#39;函数内，交换前 $a = &#39;.$a.&#39;, $b = &#39;.$b.&#39;<br>&#39;;
        $temp = $a;
        $a = $b;
        $b = $temp;
        echo &#39;函数内，交换后 $a = &#39;.$a.&#39;, $b = &#39;.$b.&#39;<br>&#39;;
    }
    $x = 5;
    $y = 7;
    echo &#39;函数外，交换前 $x = &#39;.$x.&#39;, $y = &#39;.$y.&#39;<br>&#39;;
    swap($x, $y);
    echo &#39;函数外，交换后 $x = &#39;.$x.&#39;, $y = &#39;.$y;
?>

The running results are as follows:

#It can be seen from the running results that within the function, the values ​​​​are indeed exchanged, but outside the function, the values ​​​​are not no change. So we can say that passing a function by value is just passing a copy of the variable. So if you want the function to be able to operate on external parameters of the function, you need to use reference passing.

Passing by reference

Passing by reference is to copy the memory address of the actual parameter and then pass it to the formal parameter of the function. Both parameters and formal parameters point to the same memory address, so the function's operation on the formal parameters will affect the actual parameters outside the function.

Passing by reference is to pass the memory address of the actual parameter to the formal parameter of the function. Therefore, the actual parameters and formal parameters point to the same memory address. At this time, all operations inside the function will affect the values ​​of the actual parameters outside the function. The method of passing by reference is to add an & symbol on the basis of value passing, as shown below:

function name (&参数1, &参数2, ..., &参数3) {
    ...    
}

[Example] Slightly adjust the code of the above example and use the method of passing by reference to pass to the swap function. Parameters, the code is as follows:

<?php
    function swap(&$a, &$b){
        echo &#39;函数内，交换前 $a = &#39;.$a.&#39;, $b = &#39;.$b.&#39;<br>&#39;;
        $temp = $a;
        $a = $b;
        $b = $temp;
        echo &#39;函数内，交换后 $a = &#39;.$a.&#39;, $b = &#39;.$b.&#39;<br>&#39;;
    }
    $x = 5;
    $y = 7;
    echo &#39;函数外，交换前 $x = &#39;.$x.&#39;, $y = &#39;.$y.&#39;<br>&#39;;
    swap($x, $y);
    echo &#39;函数外，交换后 $x = &#39;.$x.&#39;, $y = &#39;.$y;
?>

The running results are as follows:

The difference between php value passing and reference passing

Value transfer: within the scope of the function, changing the value of the variable will not affect the value of the variable outside the function.

Passing by reference: Within the function scope, any changes to the value are also reflected outside the function, because the memory address is passed by reference.

Type two pieces of code and you can see the difference between the two. Let’s look at the essence through the phenomenon

function sum($a){
  $a++;
  $b = $a;
  return $b;
}
$a = 10;
echo sum($a).&#39;<br />&#39;;//11
echo $a;//10

function sum(&$a){
  $a++;
  $b = $a;
  return $b;
}
$a = 10;
echo sum($a).&#39;<br />&#39;;//11
echo $a;//11

The difference between the two pieces of code lies in the parameters of the function sum. One is to pass the value $ a, and the other is passing reference &$a. The result is that the value of $a does not change after passing the value. On the contrary, the value of $a changes after passing the reference. Children who have learned C language here will understand what is going on. What is pushed on the stack is a copy of the reference.

Since the reference points to a certain variable, the operation on the reference is actually the operation on the variable it points to. (The function is the same as passing a pointer, except that there is no need to dereference) & is a symbol pointing to the address of a variable. The formal parameter &$a in the function sum is actually the actual parameter $a, so it is passed into sum for an operation. After that, the value of the actual parameter $a actually changes, that's all.

Explanation:

The value of the original parameter in pass by value remains the original value after calling other functions, while pass by reference changes the original value. When passing values ​​according to the method of passing by value, if the original value needs to be changed, the code needs to be copied. If the value is large enough or it is a large string of strings, the code will be more and more repetitive, while passing by reference will There is no need to copy PHP code, which has a great advantage in improving performance.

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