Garbage collection mechanism of PHP functions

PHP uses a generational garbage collector to automatically reclaim memory through reference counting and mark cleaning. PHP keeps track of the number of references pointing to a variable, and when the reference count reaches 0, the variable is considered no longer in use. PHP marks all reachable objects starting from the root node, and unmarked objects will be cleared to free up memory. Manual memory management techniques include explicitly destroying variables using unset(), avoiding reference cycles, and using weak references.

Garbage collection mechanism of PHP function

Introduction

PHP uses generation A garbage collector that automatically reclaims memory that is no longer in use by using techniques such as reference counting and mark-and-sweep.

Reference Counting

When a variable is created, PHP allocates a reference counter that records the number of variables pointing to it. When a variable goes out of scope, its reference count is decremented. When the reference count reaches 0, the variable is considered no longer in use.

Mark clearing

During a garbage collection cycle, PHP will mark all reachable objects starting from the root node (such as global variables and variables that are still in use). After marking is complete, PHP clears the unmarked objects and frees the memory they occupy.

Manual Memory Management

Although PHP automatically collects garbage, it is also useful to know some manual memory management techniques. Here are some methods:

• Reference counting optimization: Use unset() to explicitly destroy variables that are no longer needed.
• Reference cycle: Avoid creating a circular reference where the reference variable points to itself, which may lead to memory leaks.
• Weak reference: Use WeakMap to store lightweight data that does not prevent its associated variables from being recycled.

Practical case

The following code shows how to use unset() to manually destroy variables that are no longer needed:

<?php
function foo() {
    $a = 'foo';
    // ... 使用 $a ...
    unset($a); // 显式销毁 $a
}

// 调用 foo() 释放 $a 占用的内存
foo();

Conclusion

PHP’s garbage collection mechanism helps improve application performance and stability by automatically releasing memory that is no longer used. By understanding the reference counting and mark clearing process, as well as mastering manual memory management techniques, developers can further optimize their application's memory usage.

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