Can each class in php be loaded only once?

In the PHP development process, we often use classes to encapsulate code to achieve better organization and reuse. For a class, we only need to introduce it where needed, and then we can create objects and use them. But, have you ever thought that each class only needs to be loaded once? Otherwise, what will be the consequences?

First, let us take a look at the process of class loading. In PHP, we can use functions such as require or include to load a class file, for example:

require_once 'path/to/MyClass.php';

When this statement is executed, PHP will read the code in the MyClass.php file and The classes defined therein are parsed and compiled. When we call this class again and create an object elsewhere, PHP will find that this class has been parsed and compiled, so it will use the previous results directly without parsing and compiling.

This mechanism seems to be very efficient, because each class only needs to be processed once. But what happens if we try to load a class multiple times in the same script? Let's give it a try:

require_once 'path/to/MyClass.php';
require_once 'path/to/MyClass.php';
$obj1 = new MyClass();
$obj2 = new MyClass();
echo ($obj1 === $obj2) ? 'same' : 'different';

In this example, we load the same class file twice in a row and create two objects. Then we determine whether the two objects are the same, and if so, output 'same', otherwise output 'different'.

If the class can only be loaded once, we would expect to output 'same', but in fact it outputs 'different'! This is because when the same class is loaded for the second time, PHP will ignore the previously compiled results and recompile it, resulting in that the final object created is not the same instance.

This result looks very strange, but it is actually not difficult to understand. Since PHP is a dynamic language, we can modify the definitions of variables, functions, and classes at any time. Therefore, in order to ensure program correctness and consistency, PHP must parse and compile each class at runtime.

However, this mechanism also brings some problems. Since the same class may be loaded multiple times, its code may also be executed multiple times. Not only does this waste system resources, it may also lead to some unexpected consequences. For example, in some cases, we need to define global variables or perform certain initialization operations in class files, and these operations will also be performed multiple times, resulting in uncertain program behavior.

In order to solve these problems, we can use a method similar to the automatic loading mechanism to only load each class once. PHP provides a class autoloading mechanism, which can register a loader function (loader) into the autoloading stack through the spl_autoload_register function. When a class needs to be loaded, PHP will call these loader functions sequentially until an available one is found. up to the category. In this way, when we need to use a certain class, PHP will automatically load it for us, and will only load it once. This method greatly reduces the waste of resources during loading and compilation, and improves the performance and maintainability of the program.

In short, in PHP, it is a very important concept that a class can only be loaded once. By understanding this concept, we can better understand the class loading and compilation process and avoid unnecessary code duplication and resource waste.

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