Usage and examples of function keyword in PHP

The function keyword in PHP is used to define functions. A function is a reusable block of code that accepts input parameters and returns a value. Use the function keyword to create your own functions in PHP, making the code more modular and maintainable.

The basic syntax of the function keyword is as follows:

function 函数名(参数1, 参数2, ...) {
  // 函数体
  return 值;
}

The function name can be customized, but it needs to follow certain naming rules, such as it can only contain letters and numbers, and cannot start with a number, etc.

Parameters can be of any type, including scalar types (such as integers, floating point numbers, strings, Boolean values, etc.), arrays, and objects. There can be multiple parameters, separated by commas.

The function body is the code block for function execution and can contain any legal PHP code. In the function body, parameters can be used to perform calculations, operations, and control processes.

The return value refers to the value returned to the caller after the function is executed, and can be of any type. Just use the return keyword followed by the value to be returned.

The following is an example of using the function keyword to define a function:

function square($num) {
  return $num * $num;
}

$result = square(5); // 调用函数，并将返回值赋给$result变量
echo $result; // 输出: 25

This example defines a function named square, which accepts a parameter $num and returns the square of the parameter value. When calling the function, pass in parameter 5 and assign the return value to the $result variable. Finally, use the echo statement to output the value of $result, and the result is 25.

In addition to simple parameter transfer and return values, functions can also have default parameters and variable-length parameters.

The default parameter is to assign an initial value to the parameter when defining the function. When the corresponding parameter is not passed in when the function is called, the default value will be used.

The following is an example of using default parameters:

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

greet(); // 输出: Hello, guest!
greet("John"); // 输出: Hello, John!

In this example, a function named greet is defined, which accepts a parameter $name, which has a default value "guest" ". When calling this function, if no parameters are passed in, the default value is used; if a parameter is passed in, the passed parameter value is used.

Variable length parameters means that any number of parameters can be accepted. When defining a function, adding three dots (...) before the parameter name indicates that the parameter can accept multiple values ​​and store them in an array.

The following is an example of using variable-length parameters:

function sum(...$nums) {
  $result = 0;
  foreach($nums as $num) {
    $result += $num;
  }
  return $result;
}

$total = sum(1, 2, 3, 4); // 调用函数，并将返回值赋给$total变量
echo $total; // 输出: 10

In this example, a function named sum is defined, which accepts any number of parameters and adds these parameters to get a sum. In the function body, use a foreach loop to traverse the parameter array $nums, accumulate the value of each parameter into the $result variable, and finally return the value of $result.

Through the usage and examples of the function keyword, we can customize functions to encapsulate complex logic and improve code reusability and readability. In actual development, functions are one of the very important and commonly used syntax features in PHP.

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