How is the bottom layer of php array implemented?

PHP is a popular programming language, especially widely used in web development. In PHP, array is a very important data structure that can be used to store and manipulate data. However, many PHP developers do not understand the internal implementation of arrays. This article will delve into the underlying implementation of PHP arrays to help developers better use and optimize PHP arrays.

1. Basic introduction to PHP arrays

In PHP, an array is an unordered, variable-length data container that can store any type of data. There are two types of PHP arrays: indexed arrays and associative arrays. Indexed arrays use numbers as indices to access elements, while associative arrays use strings as indices to access elements. The following is how two types of arrays are defined:

$indexArray = array('apple', 'orange', 'banana');
$assocArray = array('name' => 'Tom', 'age' => 18);

The way to access array elements is as follows:

$indexArray[0] // 访问索引为0的元素
$assocArray['name'] // 访问键为'name'的元素

Arrays also support operations such as adding, modifying, and deleting elements:

$indexArray[] = 'grape'; // 添加一个新元素
$indexArray[0] = 'cherry'; // 修改索引为0的元素
unset($indexArray[1]); // 删除索引为1的元素

2. Internal implementation of PHP array

The underlying implementation of PHP array is HashTable. HashTable is a hash table, its function is to map key-value pairs to a specific index. The PHP array uses a C language structure to implement HashTable. The following is its structure:

typedef struct _hashtable {
   unsigned int nTableMask;
   Bucket *arBuckets;
   unsigned int nNumOfElements;
   unsigned int nNextFreeElement;
   dtor_func_t pDestructor;
   zend_bool persistent;
   unsigned char nApplyCount;
   zend_bool bApplyProtection;
#ifdef ZEND_HASH_STATISTICS
   ulong       nTableSize;
   ulong       nTableMaskUsed;
   uint        nNumOfCollisions;
   uint        nNumOfChecks;
   uint        nNumOfInserts;
   uint        nNumOfInconsistentInserts;
   uint        nNumOfFailedExpands;
#endif/*ZEND_HASH_STATISTICS*/
} HashTable;

In the above structure, nTableMask represents the size of the hash table, and arBuckets is a Bucket array that stores all data. Bucket is a linked list structure used to resolve hash conflicts. nNumOfElements represents the number of elements in the hash table, and nNextFreeElement represents the index of the next free element. pDestructor is a callback function that handles the value of an element when it is deleted. persistent indicates whether the hash table is persistent. nApplyCount and bApplyProtection are used to support concurrent access. ZEND_HASH_STATISTICS is some statistical information used for debugging.

The underlying implementation of PHP arrays can be divided into three parts:

1. Hash function

The hash function maps array keys to hashes The key to the index in the table. PHP arrays use a number of different hash functions to ensure that the hashing is as even as possible. Hash functions typically compute a hash value using an array key and then compress the value to fit within the size of the hash table. The following are the hash functions used by PHP arrays:

ZEND_HASH_FUNC(joaat)
ZEND_HASH_FUNC(fnv)
ZEND_HASH_FUNC(djb2)
ZEND_HASH_FUNC(php)
ZEND_HASH_FUNC(sha1)

2. Access operations

The access operations of PHP arrays usually include operations such as querying, adding, modifying, and deleting elements. . When accessing an element, the PHP array first uses a hash function to calculate the hash value of the element, and then finds the corresponding Bucket based on this hash value. If this Bucket already has elements, the PHP array will traverse the entire linked list to find the corresponding elements. If the element is found, its value is returned directly. Otherwise, use nNextFreeElement to insert a new element, create a new Bucket on the appropriate Bucket, and insert the new element into the end of the linked list.

3. Garbage collection

Garbage collection of PHP arrays is usually implemented through destructors. When an element is deleted, if the element's value is a PHP object, its destructor will be called. This destructor is responsible for releasing the memory occupied by this object. If this array is stored persistently, PHP will not delete it from memory after the script is executed, but will wait until the entire PHP process ends before destroying the array.

3. Performance optimization of PHP arrays

Arrays are a very commonly used data structure in PHP, and its performance is closely related to the quality and design of the code. The following are some optimization suggestions for PHP array performance:

1. Avoid accessing the same element multiple times

Accessing elements in an array usually requires hash value calculation and linked list traversal operations, these operations will take a lot of time. When you need to access the same element multiple times, you can store its value directly in a variable to avoid calculating hash values ​​and traversing the linked list multiple times.

2. Reduce the number of array operations as much as possible

When accessing, modifying, or adding elements, try to minimize the number of array operations. Use variables to replace the elements in the array to perform calculations, and finally perform an assignment operation on the array.

3. When using unset() to delete elements, try to specify the index

When using unset() to delete elements in the array, try to specify the index to be deleted. In this way, the PHP array does not have to traverse all the elements from the beginning, and can directly find the element to be deleted.

4. Choose the correct array type

The underlying implementation principles of indexed arrays and associative arrays are different, and better performance can be obtained by using the respective array types.

5. Avoid array type conversion

In PHP, converting an array from one type to another will cause performance degradation, because the conversion requires rehashing, etc. operate.

To sum up, PHP array is a very important data structure and is widely used in PHP programming. Understanding the details of the underlying implementation of PHP arrays is important for both performance and debugging. It should be noted that the performance optimization of PHP arrays needs to be practiced according to specific application scenarios. Only by flexibly using various PHP array features can better performance optimization be achieved.

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