How to classify PHP functions in depth?

PHP functions are divided into four categories: built-in functions, user-defined functions, core functions and user-defined extension functions. Built-in functions can be used without loading, such as the echo function. User-defined functions are created by users, such as the greet function. Core functions are used for low-level system operations, such as the file_get_contents function. User-defined extension functions are provided by extensions, such as my_extension. Practical examples include using built-in functions to get the length of a string, using user-defined functions to format dates, and using core functions to read file contents.

In-depth classification of PHP functions and their practical cases

The functions in PHP are particularly rich, and they can be divided into the following categories ：

1. Built-in functions

These functions come with PHP and can be used without additional loading. For example:

echo("Hello World!");

2. User-defined functions

These functions are created by users themselves and need to be defined using the function keyword. For example:

function greet($name) {
  echo("Hello, $name!");
}

greet("John Doe");

3. Core functions

These functions are part of the PHP core extension. They are usually used to handle low-level system operations such as file operations, error handling, etc. For example:

file_get_contents("file.txt");

4. User-defined extension functions

These functions are provided by user-created extensions and require the use of the extension_loaded() function load. For example:

extension_loaded("my_extension");

Practical case

Example 1: Use built-in function to get the string length

$str = "Hello World!";
$length = strlen($str);

Example 2: Use user-defined function to format date

function format_date($date) {
  return date("Y-m-d H:i:s", $date);
}

echo format_date(time());

Example 3: Use core function to read file content

$content = file_get_contents("file.txt");

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