函数的参数

通过参数列表可以传递信息到函数，即以逗号作为分隔符的表达式列表。参数是从左向右求值的。

PHP 支持按值传递参数（默认），通过引用传递参数 以及 默认参数。也支持 可变长度参数列表 和 命名参数。

Example #1 向函数传递数组

<?php
function takes_array($input)
{
    echo "$input[0] + $input[1] = ", $input[0]+$input[1];
}
?>

从 PHP 8.0.0 开始，函数参数列表可以包含一个尾部的逗号，这个逗号将被忽略。这在参数列表较长或包含较长的变量名的情况下特别有用，这样可以方便地垂直列出参数。

Example #2 函数参数使用尾部逗号

<?php
function takes_many_args(
    $first_arg,
    $second_arg,
    $a_very_long_argument_name,
    $arg_with_default = 5,
    $again = 'a default string', // 在 8.0.0 之前，这个尾部的逗号是不允许的。
)
{
    // ...
}
?>

As of PHP 8.0.0, passing mandatory arguments after optional arguments is deprecated. This can generally be resolved by dropping the default value. One exception to this rule are arguments of the form Type $param = null, where the null default makes the type implicitly nullable. This usage remains allowed, though it is recommended to use an explicit nullable type instead.

Example #3 Passing optional arguments after mandatory arguments

<?php
function foo($a = [], $b) {} // 之前
function foo($a, $b) {}      // 之后

function bar(A $a = null, $b) {} // 同时可用
function bar(?A $a, $b) {}       // 官方推荐的写法
?>

通过引用传递参数

默认情况下，函数参数通过值传递（因而即使在函数内部改变参数的值，它并不会改变函数外部的值）。如果希望允许函数修改它的参数值，必须通过引用传递参数。

如果想要函数的一个参数总是通过引用传递，可以在函数定义中该参数的前面加上符号 &：

Example #4 用引用传递函数参数

<?php
function add_some_extra(&$string)
{
    $string .= 'and something extra.';
}
$str = 'This is a string, ';
add_some_extra($str);
echo $str;    // outputs 'This is a string, and something extra.'
?>

默认参数的值

函数可以定义 C++ 风格的标量参数默认值，如下所示：

Example #5 在函数中使用默认参数

<?php
function makecoffee($type = "cappuccino")
{
    return "Making a cup of $type.\n";
}
echo makecoffee();
echo makecoffee(null);
echo makecoffee("espresso");
?>

以上例程会输出：

Making a cup of cappuccino.
Making a cup of .
Making a cup of espresso.

PHP 还允许使用数组 array 和特殊类型 null 作为默认参数，例如：

Example #6 使用非标量类型作为默认参数

<?php
function makecoffee($types = array("cappuccino"), $coffeeMaker = NULL)
{
    $device = is_null($coffeeMaker) ? "hands" : $coffeeMaker;
    return "Making a cup of ".join(", ", $types)." with $device.\n";
}
echo makecoffee();
echo makecoffee(array("cappuccino", "lavazza"), "teapot");
?>

默认值必须是常量表达式，不能是诸如变量，类成员，或者函数调用等。

注意当使用默认参数时，任何默认参数必须放在任何非默认参数的右侧；否则，函数将不会按照预期的情况工作。考虑下面的代码片断：

Example #7 函数默认参数的不正确用法

<?php
function makeyogurt($type = "acidophilus", $flavour)
{
    return "Making a bowl of $type $flavour.\n";
}

echo makeyogurt("raspberry");   // won't work as expected
?>

以上例程会输出：

Warning: Missing argument 2 in call to makeyogurt() in
/usr/local/etc/httpd/htdocs/phptest/functest.html on line 41
Making a bowl of raspberry .

现在，比较上面的例子和这个例子：

Example #8 函数默认参数正确的用法

<?php
function makeyogurt($flavour, $type = "acidophilus")
{
    return "Making a bowl of $type $flavour.\n";
}

echo makeyogurt("raspberry");   // works as expected
?>

以上例程会输出：

Making a bowl of acidophilus raspberry.

Note: 传引用的参数也可以有默认值。

可变数量的参数列表

PHP 在用户自定义函数中支持可变数量的参数列表。由 ... 语法实现。

Note: 还可以使用以下函数来获取可变参数 func_num_args()、 func_get_arg() 和 func_get_args()，不建议使用此方式，请使用 ... 来替代。

包含 ... 的参数，会转换为指定参数变量的一个数组，见以下示例：

Example #9 使用 ... 来访问变量参数

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

echo sum(1, 2, 3, 4);
?>

以上例程会输出：

10

也可以使用 ... 语法来传递 array 或 Traversable 做为参数到函数中：

Example #10 使用 ... 来传递参数

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

echo add(...[1, 2])."\n";

$a = [1, 2];
echo add(...$a);
?>

以上例程会输出：

3
3

你可以在 ... 前指定正常的位置参数。在这种情况下，只有不符合位置参数的尾部参数才会被添加到 ... 生成的数组中。

你也可以在 ... 标记前添加一个 类型声明。如果存在这种情况，那么 ... 捕获的所有参数必须是提示类的对象。

Example #11 输入提示的变量参数

<?php
function total_intervals($unit, DateInterval ...$intervals) {
    $time = 0;
    foreach ($intervals as $interval) {
        $time += $interval->$unit;
    }
    return $time;
}

$a = new DateInterval('P1D');
$b = new DateInterval('P2D');
echo total_intervals('d', $a, $b).' days';

// This will fail, since null isn't a DateInterval object.
echo total_intervals('d', null);
?>

以上例程会输出：

3 days
Catchable fatal error: Argument 2 passed to total_intervals() must be an instance of DateInterval, null given, called in - on line 14 and defined in - on line 2

最后，你还可以给参数传递 引用变量，通过在 ... 前加上一个 (&) 符号来实现。

旧版本的 PHP

不需要特殊的语法来声明一个函数是可变的；但是访问函数的参数必须使用 func_num_args(), func_get_arg() 和 func_get_args() 函数。

上面的第一个例子在早期 PHP 版本中的实现如下：

Example #12 在 PHP 早期版本中访问可变参数

<?php
function sum() {
    $acc = 0;
    foreach (func_get_args() as $n) {
        $acc += $n;
    }
    return $acc;
}

echo sum(1, 2, 3, 4);
?>

以上例程会输出：

10

命名参数

PHP 8.0.0 开始引入了命名参数作为现有位置参数的扩展。命名参数允许根据参数名而不是参数位置向函数传参。这使得参数的含义自成体系，参数与顺序无关，并允许任意跳过默认值。

命名参数通过在参数名前加上冒号来传递。允许使用保留关键字作为参数名。参数名必须是一个标识符，不允许动态指定。

Example #13 命名参数的语法

<?php
myFunction(paramName: $value);
array_foobar(array: $value);

// NOT supported.
function_name($variableStoringParamName: $value);
?>

Example #14 通过位置传参与命名参数的对比

<?php
// 使用顺序传递参数：
array_fill(0, 100, 50);

// 使用命名参数：
array_fill(start_index: 0, count: 100, value: 50);
?>

指定参数的传递顺序并不重要。

Example #15 参数顺序不同的示例（同上例）

<?php
array_fill(value: 50, num: 100, start_index: 0);
?>

命名参数也可以与位置参数相结合使用。此种情况下，命名参数必须在位置参数之后。也可以只指定一个函数的部分可选参数，而不考虑它们的顺序。

Example #16 命名参数与位置参数结合使用

<?php
htmlspecialchars($string, double_encode: false);
// 等价于
htmlspecialchars($string, ENT_COMPAT | ENT_HTML401, 'UTF-8', false);
?>

Passing the same parameter multiple times results in an Error exception.

Example #17 Error exception when passing the same parameter multiple times

<?php
function foo($param) { ... }

foo(param: 1, param: 2);
// Error: Named parameter $param overwrites previous argument
foo(1, param: 2);
// Error: Named parameter $param overwrites previous argument
?>

User Contributed Notes

rsperduta at gmail dot com 13-Feb-2021 07:33
About example #2: That little comma down at the end and often obscured by a line comment is easily over looked. I think it's worth considering putting it at the head of the next line to make clear what it's relationship is to the surrounding lines. Consider how much clearer it's continuation as a list of parameters:

<?php
function takes_many_args(
    $first_arg // some description
    , $second_arg // another comment
    , $a_very_long_argument_name = something($complicated) // IDK
    , $arg_with_default = 5
    , $again = 'a default string', // IMHO this trailing comma encourages illegible code and not being permitted seemed  a good idea lost with 8.0.0.
) {
    // ...
}
?>

 This principle can be applied equally to complicated boolean expressions of an "if" statement (or the parts of a for statement).
shaman_master at list dot ru 22-Nov-2019 04:38
You can use the class/interface as a type even if the class/interface is not  defined yet or the class/interface implements current class/interface.
<?php
interface RouteInterface
{
    public function getGroup(): ?RouteGroupInterface;
}
interface RouteGroupInterface extends RouteInterface
{
    public function set(RouteInterface $item);
}
?>
'self' type - alias to current class/interface, it's not changed in implementations. This code looks right but throw error:
<?php
class Route
{
    protected $name;
     // method must return Route object
    public function setName(string $name): self
    {
         $this->name = $name;
         return $this;
    }
}
class RouteGroup extends Route
{
    // method STILL must return only Route object
    public function setName(string $name): self
    {
         $name .= ' group';
         return parent::setName($name);
    }
}
?>
dmitry dot balabka at gmail dot com 13-Nov-2018 09:47
There is a possibility to use parent keyword as type hint which is not mentioned in the documentation.

Following code snippet will be executed w/o errors on PHP version 7. In this example, parent keyword is referencing on ParentClass instead of ClassTrait.
<?php
namespace TestTypeHints;

class ParentClass
{
    public function someMethod()
    {
        echo 'Some method called' . \PHP_EOL;
    }
}

trait ClassTrait
{
    private $original;

    public function __construct(parent $original)
    {
        $this->original = $original;
    }

    protected function getOriginal(): parent
    {
        return $this->original;
    }
}

class Implementation extends ParentClass
{
    use ClassTrait;

    public function callSomeMethod()
    {
        $this->getOriginal()->someMethod();
    }
}

$obj = new Implementation(new ParentClass());
$obj->callSomeMethod();
?>

Outputs:
Some method called
boan dot web at outlook dot com 08-Feb-2018 11:27
Quote:

"The declaration can be made to accept NULL values if the default value of the parameter is set to NULL."

But you can do this (PHP 7.1+):

<?php
function foo(?string $bar) {
    //...
}

foo(); // Fatal error
foo(null); // Okay
foo('Hello world'); // Okay
?>
igorsantos07 at gmail dot com 27-Dec-2017 05:06
PHP 7+ does type coercion if strict typing is not enabled, but there's a small gotcha: it won't convert null values into anything.

You must explicitly set your default argument value to be null (as explained in this page) so your function can also receive nulls.

For instance, if you type an argument as "string", but pass a null variable into it, you might expect to receive an empty string, but actually, PHP will yell at you a TypeError.

<?php
function null_string_wrong(string $str) {
  var_dump($str);
}
function null_string_correct(string $str = null) {
  var_dump($str);
}
$null = null;
null_string_wrong('a');     //string(1) "a"
null_string_correct('a');   //string(1) "a"
null_string_correct();      //NULL
null_string_correct($null); //NULL
null_string_wrong($null);   //TypeError thrown
?>
info at keraweb dot nl 21-Sep-2017 09:25
You can use a class constant as a default parameter.

<?php

class A {
    const FOO = 'default';
    function bar( $val = self::FOO ) {
        echo $val;
    }
}

$a = new A();
$a->bar(); // Will echo "default"
Hayley Watson 30-Aug-2017 11:41
There are fewer restrictions on using ... to supply multiple arguments to a function call than there are on using it to declare a variadic parameter in the function declaration. In particular, it can be used more than once to unpack arguments, provided that all such uses come after any positional arguments.

<?php

$array1 = [[1],[2],[3]];
$array2 = [4];
$array3 = [[5],[6],[7]];

$result = array_merge(...$array1); // Legal, of course: $result == [1,2,3];
$result = array_merge($array2, ...$array1); // $result == [4,1,2,3]
$result = array_merge(...$array1, $array2); // Fatal error: Cannot use positional argument after argument unpacking.
$result = array_merge(...$array1, ...$array3); // Legal! $result == [1,2,3,5,6,7]
?>

The Right Thing for the error case above would be for $result==[1,2,3,4], but this isn't yet (v7.1.8) supported.
Hayley Watson 16-Aug-2017 12:50
If you use ... in a function's parameter list, you can use it only once for obvious reasons. Less obvious is that it has to be on the LAST parameter; as the manual puts it: "You may specify normal positional arguments BEFORE the ... token. (emphasis mine).

<?php
function variadic($first, ...$most, $last)
{/*etc.*/}

variadic(1, 2, 3, 4, 5);
?>
results in a fatal error, even though it looks like the Thing To Do™ would be to set $first to 1, $most to [2, 3, 4], and $last to 5.
gabriel at figdice dot org 11-Aug-2016 07:05
A function's argument that is an object, will have its properties modified by the function although you don't need to pass it by reference.

<?php
$x = new stdClass();
$x->prop = 1;

function f ( $o ) // Notice the absence of &
{
  $o->prop ++;
}

f($x);

echo $x->prop; // shows: 2
?>

This is different for arrays:

<?php
$y = [ 'prop' => 1 ];

function g( $a )
{
  $a['prop'] ++;
  echo $a['prop'];  // shows: 2
}

g($y);

echo $y['prop'];  // shows: 1
?>
catman at esteticas dot se 07-Feb-2016 01:06
I wondered if variable length argument lists and references works together, and what the syntax might be. It is not mentioned explicitly yet in the php manual as far as I can find. But other sources mention the following syntax "&...$variable" that works in php  5.6.16.

<?php
function foo(&...$args)
{
    $i = 0;
    foreach ($args as &$arg) {
        $arg = ++$i;
    }
}
foo($a, $b, $c);
echo 'a = ', $a, ', b = ', $b, ', c = ', $c;
?>
Gives
a = 1, b = 2, c = 3
php at richardneill dot org 28-Mar-2015 07:24
To experiment on performance of pass-by-reference and pass-by-value, I used this  script. Conclusions are below.

#!/usr/bin/php
<?php
function sum($array,$max){   //For Reference, use:  "&$array"
    $sum=0;
    for ($i=0; $i<2; $i++){
        #$array[$i]++;        //Uncomment this line to modify the array within the function.
        $sum += $array[$i]; 
    }
    return ($sum);
}

$max = 1E7                  //10 M data points.
$data = range(0,$max,1);

$start = microtime(true);
for ($x = 0 ; $x < 100; $x++){
    $sum = sum($data, $max);
}
$end =  microtime(true);
echo "Time: ".($end - $start)." s\n";

/* Run times:
#    PASS BY    MODIFIED?   Time
-    -------    ---------   ----
1    value      no          56 us
2    reference  no          58 us

3    valuue     yes         129 s
4    reference  yes         66 us

Conclusions:

1. PHP is already smart about zero-copy / copy-on-write. A function call does NOT copy the data unless it needs to; the data is
   only copied on write. That's why  #1 and #2 take similar times, whereas #3 takes 2 million times longer than #4.
   [You never need to use &$array to ask the compiler to do a zero-copy optimisation; it can work that out for itself.]

2. You do use &$array  to tell the compiler "it is OK for the function to over-write my argument in place, I don't need the original
   any more." This can make a huge difference to performance when we have large amounts of memory to copy.
   (This is the only way it is done in C, arrays are always passed as pointers)

3. The other use of & is as a way to specify where data should be *returned*. (e.g. as used by exec() ).
   (This is a C-like way of passing pointers for outputs, whereas PHP functions normally return complex types, or multiple answers
   in an array)

4. It's  unhelpful that only the function definition has &. The caller should have it, at least as syntactic sugar. Otherwise
   it leads to unreadable code: because the person reading the function call doesn't expect it to pass by reference. At the moment,
   it's necessary to write a by-reference function call with a comment, thus:
    $sum = sum($data,$max);  //warning, $data passed by reference, and may be modified.

5. Sometimes, pass by reference could be at the choice of the caller, NOT the function definitition. PHP doesn't allow it, but it
   would be meaningful for the caller to decide to pass data in as a reference. i.e. "I'm done with the variable, it's OK to stomp
   on it in memory".
*/
?>
Horst Schirmeier 08-Jan-2014 11:58
Editor's note: what is expected here by the parser is a non-evaluated expression. An operand and two constants requires evaluation, which is not done by the parser. However, this feature is included as of PHP 5.6.0. See this page for more information: http://php.net/migration56.new-features#migration56.new-features.const-scalar-exprs
--------

"The default value must be a constant expression" is misleading (or even wrong).  PHP 5.4.4 fails to parse this function definition:

function htmlspecialchars_latin1($s, $flags = ENT_COMPAT | ENT_HTML401) {}

This yields a " PHP Parse error:  syntax error, unexpected '|', expecting ')' " although ENT_COMPAT|ENT_HTML401 is certainly what a compiler-affine person would call a "constant expression".

The obvious workaround is to use a single special value ($flags = NULL) as the default, and to set it to the desired value in the function's body (if ($flags === NULL) { $flags = ENT_COMPAT | ENT_HTML401; }).
John 15-Nov-2006 03:20
This might be documented somewhere OR obvious to most, but when passing an argument by reference (as of PHP 5.04) you can assign a value to an argument variable in the function call. For example:

function my_function($arg1, &$arg2) {
  if ($arg1 == true) {
    $arg2 = true;
  }
}
my_function(true, $arg2 = false);
echo $arg2;

outputs 1 (true)

my_function(false, $arg2 = false);
echo $arg2;

outputs 0 (false)
jcaplan at bogus dot amazon dot com 09-Mar-2006 03:11
In function calls, PHP clearly distinguishes between missing arguments and present but empty arguments.  Thus:

<?php
function f( $x = 4 ) { echo $x . "\\n"; }
f(); // prints 4
f( null ); // prints blank line
f( $y ); // $y undefined, prints blank line
?>

The utility of the optional argument feature is thus somewhat diminished.  Suppose you want to call the function f many times from function g, allowing the caller of g to specify if f should be called with a specific value or with its default value:

<?php
function f( $x = 4 ) {echo $x . "\\n"; }

// option 1: cut and paste the default value from f's interface into g's
function g( $x = 4 ) { f( $x ); f( $x ); }

// option 2: branch based on input to g
function g( $x = null ) { if ( !isset( $x ) ) { f(); f() } else { f( $x ); f( $x ); } }
?>

Both options suck.

The best approach, it seems to me, is to always use a sentinel like null as the default value of an optional argument.  This way, callers like g and g's clients have many options, and furthermore, callers always know how to omit arguments so they can omit one in the middle of the parameter list.

<?php
function f( $x = null ) { if ( !isset( $x ) ) $x = 4; echo $x . "\\n"; }

function g( $x = null ) { f( $x ); f( $x ); }

f(); // prints 4
f( null ); // prints 4
f( $y ); // $y undefined, prints 4
g(); // prints 4 twice
g( null ); // prints 4 twice
g( 5 ); // prints 5 twice

?>
Sergio Santana: ssantana at tlaloc dot imta dot mx 31-Oct-2005 02:59
PASSING A "VARIABLE-LENGTH ARGUMENT LIST OF REFERENCES" TO A FUNCTION
As of PHP 5, Call-time pass-by-reference has been deprecated, this represents no problem in most cases, since instead of calling a function like this:
   myfunction($arg1, &$arg2, &$arg3);

you can call it
   myfunction($arg1, $arg2, $arg3);

provided you have defined your function as
   function myfuncion($a1, &$a2, &$a3) { // so &$a2 and &$a3 are
                                                             // declared to be refs.
    ... <function-code>
   }

However, what happens if you wanted to pass an undefined number of references, i.e., something like:
   myfunction(&$arg1, &$arg2, ..., &$arg-n);?
This doesn't work in PHP 5 anymore.

In the following code I tried to amend this by using the
array() language-construct as the actual argument in the
call to the function.

<?php

  function aa ($A) {
    // This function increments each
    // "pseudo-argument" by 2s
    foreach ($A as &$x) {
      $x += 2;
    }
  }
 
  $x = 1; $y = 2; $z = 3;
 
  aa(array(&$x, &$y, &$z));
  echo "--$x--$y--$z--\n";
  // This will output:
  // --3--4--5--
?>

I hope this is useful.

Sergio.