What is the difference between the execution order of PHP predefined functions and user-defined functions?

PHP executes functions in a specific order: 1. Predefined functions (highest priority); 2. User-defined functions (in order of declaration). Predefined functions take precedence over custom functions, for example echo() is executed before myFunction().

Execution order of PHP predefined functions and custom functions

Overview

PHP follows a specific execution order when executing functions, which affects the priority and availability of the function. Understanding the difference between predefined and user-defined functions is critical to managing your code effectively.

Predefined functions

• Built into the PHP language
• Can be used directly without any declaration
• With Highest priority

Common predefined functions include:

echo()
print()
rand()
count()

User-defined functions

• Developer-defined functions
• Needs to be declared and defined before use
• Priority is lower than predefined functions

The following example shows the declaration and definition of custom functions:

function myFunction() {
  // 函数体
}

Execution order

PHP executes functions in the following order:

1. Predefined functions
2. User-defined functions (in order of declaration)

This means that predefined functions תמיד will take precedence over user-defined functions.

Practical case

Consider the following code:

echo("预定义函数");
myFunction();

function myFunction() {
  echo("自定义函数");
}

The output will be:

预定义函数自定义函数

This indicates the predefined function echo() is executed before the user-defined function myFunction().

Conclusion

Understanding the execution order between predefined functions and user-defined functions is critical to effectively organizing code and avoiding conflicts. Predefined functions have priority, so they should be used first when special functionality is required.

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