PHP garbage collection mechanism and memory management

PHP's memory management uses a garbage collection mechanism, which automatically releases unused memory at runtime and operates based on the principle of reference counting. To avoid memory leaks, follow these best practices: destroy variables no longer needed, avoid circular references, and use weak references.

PHP garbage collection mechanism and memory management

Memory management in PHP

PHP uses a garbage collection mechanism to manage memory, which means Then it will automatically release memory that is no longer used at runtime. Garbage collection is handled by the PHP interpreter, so developers generally do not need to manually manage memory.

The principle of garbage collection

PHP’s garbage collection is based on reference counting. Each PHP variable maintains a reference counter that tracks the number of variables and objects that reference it. When the reference counter reaches 0, it means that the variable is no longer used, and the PHP interpreter will automatically release the memory occupied by the variable.

Example

To demonstrate the garbage collection mechanism, let us create a simple PHP script:

<?php
$obj = new stdClass();
$obj->name = "John Doe";

// 将对象的引用传递给另一个变量
$ref1 = $obj;
// 再将对象的引用传递给另一个变量
$ref2 = $obj;

// 销毁第一个引用
unset($ref1);

// PHP 解释器会自动释放对象占用的内存，因为现在只剩下一个引用计数

Avoid memory leaks

Although the garbage collection mechanism automatically releases memory, but there is still a possibility of memory leaks. A memory leak is when a reference to a variable or object that is no longer used is retained, preventing the PHP interpreter from releasing the memory.

To avoid memory leaks, follow these best practices:

• Destroy variables no longer needed: Use the unset() function to release references to variables .
• Avoid circular references: Two or more objects reference each other, causing the reference counter to fail to return to zero.
• Use weak references: If you need to retain an object reference but do not want to prevent garbage collection, you can use weak references.

Practical Case

Let us use the previous script to create a practical case to demonstrate the garbage collection mechanism:

<?php
class MyClass
{
    public $data;

    public function __destruct()
    {
        echo "释放 MyClass 实例\n";
    }
}

$obj = new MyClass();
$obj->data = "一些数据";

// 将对象的引用传递给另一个变量
$ref = $obj;

// 销毁第一个引用
unset($obj);

// PHP 解释器会自动释放对象占用的内存，因为现在只剩下一个引用计数
echo "脚本结束\n";

Running this script will print the following output:

释放 MyClass 实例
脚本结束

This indicates that the PHP interpreter automatically releases the memory occupied by the MyClass instance at the end of the script.

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