PHP learning function

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Function definition

1. Valid function names start with letters or underscores, followed by letters, numbers or underscores;

2. Function names can be size-insensitive (actually, PHP's function names, method names, class Names are not case-sensitive);

3. Functions do not need to be defined before calling, unless a function is defined conditionally;

4. PHP does not support function overloading, and it is not possible to undefine or redefine declared functions.

<?php
Test(); # function test.

function test()
{
    echo 'function test.';
}

Parameters of the function

1. Value transfer

Changing the value of the parameter inside the function will not affect the value outside the function Parameter value:

<?php
$i = 1;
add($i);
echo $i; # 1

function add($test)
{
    $test++;
}

2. Pass by reference

Keep the parameter value consistent inside and outside the function. To pass by reference, you can add the symbol & in front of the parameter in the function definition:

<?php
$i = 1;
add($i);
echo $i; # 2

function add(&$test)
{
    $test++;
}

3. Variable number of parameters

PHP supports a variable number of parameters . The parameters listed when defining a function are actually just the minimum parameters; multiple parameters are allowed when calling, but the extra parameters will be ignored:

function add($a, $b) 
{
    return $a + $b;
}

echo add(1,2,5); # 3

In PHP 5.5 and earlier versions, if If you need to handle an uncertain number of parameters, you can use the following function:
func_num_args() — Return the number of parameters passed to the function
func_get_arg() — Return Specified parameters
func_get_args() — Returns the parameter list as an array

<?php
function func() 
{
    $argc = func_num_args();
    $argv = func_get_args();
    switch ($argc) {
       case 0:
           echo "没有参数。<br />";
           break;
       case 1:
           echo "只有1个参数。它是：" . func_get_arg(0) . "。<br />";
           break;
       case 2:
           echo "有2个参数。它们是：" . func_get_arg(0) . "," . func_get_arg(1) . "。<br />";
           break;
       default:
           echo "超过2个参数。它们是：";
           print_r($argv);
           break;
    }
}

func();          # 没有参数。
func(1);         # 只有1个参数。它是：1。
func(1,2);       # 有2个参数。它们是：1,2。
func(1,2,3,4,5); # 超过2个参数。它们是：Array ( [0] => 1 [1] => 2 [2] => 3 [3] => 4 [4] => 5 )

In PHP 5.6 and above, the syntax is ... Implement a variable number of parameter lists:

function sum(...$numbers) 
{
    $acc = 0;
    foreach ($numbers as $n) {
        $acc += $n;
    }
    return $acc;
}

echo sum(1, 2, 3, 4); # 10

Variable function

PHP supports the concept of variable functions - if there are parentheses after a variable name, PHP will look for the same name as the variable's value function and try to execute it:

<?php
function foo() 
{
    echo "In foo()";
}

$func = 'foo';
$func(); # In foo()

Note: Variable functions do not support language structures in PHP, such as echo, print, etc.

Anonymous function

In PHP, an anonymous function, also called a closure function, allows the temporary creation of a function without a specified name. Parameters commonly used in callback functions:

<?php
print_r(array_map(function($value) {
    return $value * 2;
}, range(1, 5)));
# Array ( [0] => 2 [1] => 4 [2] => 6 [3] => 8 [4] => 10 )

Closure functions can also be used as the value of variables:

$greet = function($name) {
    printf("Hello %s\r\n", $name);
};

$greet('World'); # Hello World

Closures can inherit variables from the parent scope. Any such variables should be passed in using use language constructs:

<?php
$message = 'hello';

$example = function () use ($message) {
    echo $message;
};
$example(); # hello

If you need to delay binding the variables in use, you need to use a reference, otherwise in the definition When the time comes, a copy will be made and placed in use:

<?php
$result = 0;
 
$one = function() use ($result) { 
    var_dump($result); 
};
 
$two = function() use (&$result) { 
    var_dump($result); 
};
 
$result++;
 
$one();  # int(0)
$two();  # int(1)

Function dynamic call

In PHP, in addition to directly calling the function, you can also call it through call_user_func() or call_user_func_array() Function indirect call function:

<?php
function named_func($param1, $param2)
{
    echo "这是一个命名函数。参数是：$param1 和 $param2 。<br />";
}

call_user_func("named_func", "PARAM1", "PARAM2");

call_user_func_array("named_func", array("PARAM1", "PARAM2"));

Related recommendations:

PHP Learning Array

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