What is the difference between custom PHP functions and predefined functions?

The difference between custom PHP functions and predefined functions is: Scope: Custom functions are limited to their definition scope, while predefined functions can be accessed throughout the script. How to define: Custom functions are defined using the function keyword, while predefined functions are defined by the PHP kernel. Parameter passing: Custom functions receive parameters, while predefined functions may not require parameters. Extensibility: Custom functions can be created as needed, while predefined functions are built-in and cannot be modified.

The difference between custom PHP functions and predefined functions

Introduction
PHP provides A large number of predefined functions and a mechanism for creating custom functions. There are some key differences between these two function types, and understanding these differences is crucial to writing PHP scripts effectively.

1. Scope

• Custom functions: can only be accessed within the scope in which they are defined.
• Predefined functions: Accessible throughout PHP scripts.

2. Definition method

• Custom function: Use function keyword definition.
• Predefined functions: Defined by the PHP core.

3. Use of parameters

• Custom function: Pass parameters through the parameter list.
• Predefined functions: Does not necessarily require parameters, but they can be passed through the parameter list.

4. Extensibility

• Custom functions: can be created according to project needs.
• Predefined functions: It is built-in and cannot be customized.

Practical case: String processing

Suppose we have a string and need to convert all lowercase letters to uppercase letters. We can use PHP's strtoupper() predefined function or create our own custom function:

// 使用预定义函数
$str = strtoupper('hello world');

// 创建自定义函数
function myStrtoupper($str) {
  return strtoupper($str);
}

$str2 = myStrtoupper('hello world');

In both cases, $str and $str2 will contain the uppercase string "HELLO WORLD".

Conclusion

Understanding the difference between custom functions and predefined functions is crucial to using PHP effectively. Custom functions provide the flexibility to create functions for specific needs, while predefined functions provide extensive functionality and ready-to-use convenience.

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