There are two ways to pass parameters in PHP: by value and by reference. Passing by value is the default method. Functions operate on variable copies and do not affect the original variables; for example, modifying $num in addOne function does not affect the external $a. Passing by reference is achieved by adding an & symbol before the parameter. Operations within the function directly affect the original variable. If modifying $num will change the external $a. Reference passing is suitable for situations where original variables need to be modified, memory-saving processed by large objects, or implemented specific design patterns. However, you need to pay attention to logical complexity and avoid arbitrary use of references to basic types. PHP objects are passed in handles by default, with similar behaviors but different mechanisms. Passing by value is safer, passing by reference is more flexible but requires caution.

In PHP, there are two ways to pass function parameters: pass by value and pass by reference. Understanding their differences can help you avoid common mistakes and also allow you to write clearer and more controllable code.

What is value-based delivery?

Passing by value is the default parameter delivery method in PHP. That is to say, when you pass a variable to a function, the function operates internally on a copy of the variable and will not affect the original variable.

For example:

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

$a = 5;
addOne($a);
echo $a; // Output is still 5

In this example, the value of $a is passed to $num , and the modification of $num inside the function will not affect $a itself. The advantage of this method is that it is safe and does not accidentally modify the original data.

What is passing by reference?

Passing by reference means that the function operates internally on the original variable itself, not its copy. If you want the function to modify the variables passed in, you need to use reference passes.

In PHP, reference passing is achieved by adding & symbol before function parameters:

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

$a = 5;
addOne($a);
echo $a; // Output 6

Here, $a is passed in as a reference, and the modification of $num inside the function will directly affect $a . This method is suitable for situations where variables need to be "modified in place", such as batch processing of arrays, status updates, etc.

When should I pass it with a reference?

• You need to modify the original variable : for example, write a function to clean or modify the passed array.
• Save memory when handling large objects : Although PHP defaults to reference passing (object handle) for objects, if you pass an object array or complex structure, using references can avoid copying the entire structure.
• Implement specific design patterns : such as observer mode, callback processing, etc., and reference transfer can more conveniently share state.

But it should also be noted that reference passing will make the code logic less intuitive, especially when variables are modified in multiple places, which can easily cause confusion.

Some details to pay attention to

• When the function is defined, & is added. When calling, you don’t need to add it. Just pass the variable directly.
• Do not use references to basic types (such as int, string) casually unless you really need to modify the original value.
• PHP objects are "handle" passed by default, and behave like references, but not real reference mechanisms.

Overall, passing by value is safer and suitable for most cases; passing by reference is more flexible, but requires caution. Understanding the difference between these two can give you a more control over the writing function.

Basically that's it.

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