PHP: Function arguments

Function arguments

Information may be passed to functions via the argument list, which is a comma-delimited list of expressions. The arguments are evaluated from left to right, before the function is actually called (eager evaluation).

PHP supports passing arguments by value (the default), passing by reference, and default argument values. Variable-length argument lists and Named Arguments are also supported.

Example #1 Passing arrays to functions

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

As of PHP 8.0.0, the list of function arguments may include a trailing comma, which will be ignored. That is particularly useful in cases where the list of arguments is long or contains long variable names, making it convenient to list arguments vertically.

Example #2 Function Argument List with trailing Comma

<?php
function takes_many_args(
    $first_arg,
    $second_arg,
    $a_very_long_argument_name,
    $arg_with_default = 5,
    $again = 'a default string', // This trailing comma was not permitted before 8.0.0.
)
{
    // ...
}
?>

Passing arguments by reference

By default, function arguments are passed by value (so that if the value of the argument within the function is changed, it does not get changed outside of the function). To allow a function to modify its arguments, they must be passed by reference.

To have an argument to a function always passed by reference, prepend an ampersand (&) to the argument name in the function definition:

Example #3 Passing function parameters by reference

<?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.'
?>

It is an error to pass a value as argument which is supposed to be passed by reference.

Default argument values

A function may define default values for arguments using syntax similar to assigning a variable. The default is used only when the parameter is not specified; in particular, note that passing null does not assign the default value.

Example #4 Use of default parameters in functions

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

The above example will output:

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

Default parameter values may be scalar values, arrays, the special type null, and as of PHP 8.1.0, objects using the new ClassName() syntax.

Example #5 Using non-scalar types as default values

<?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");?>

Example #6 Using objects as default values (as of PHP 8.1.0)

<?php
class DefaultCoffeeMaker {
    public function brew() {
        return 'Making coffee.';
    }
}
class FancyCoffeeMaker {
    public function brew() {
        return 'Crafting a beautiful coffee just for you.';
    }
}
function makecoffee($coffeeMaker = new DefaultCoffeeMaker)
{
    return $coffeeMaker->brew();
}
echo makecoffee();
echo makecoffee(new FancyCoffeeMaker);
?>

The default value must be a constant expression, not (for example) a variable, a class member or a function call.

Note that any optional arguments should be specified after any required arguments, otherwise they cannot be omitted from calls. Consider the following example:

Example #7 Incorrect usage of default function arguments

<?php
function makeyogurt($container = "bowl", $flavour)
{
    return "Making a $container of $flavour yogurt.\n";
}
 
echo makeyogurt("raspberry"); // "raspberry" is $container, not $flavour
?>

The above example will output:

Fatal error: Uncaught ArgumentCountError: Too few arguments
 to function makeyogurt(), 1 passed in example.php on line 42

Now, compare the above with this:

Example #8 Correct usage of default function arguments

<?php
function makeyogurt($flavour, $container = "bowl")
{
    return "Making a $container of $flavour yogurt.\n";
}
 
echo makeyogurt("raspberry"); // "raspberry" is $flavour
?>

The above example will output:

Making a bowl of raspberry yogurt.

As of PHP 8.0.0, named arguments can be used to skip over multiple optional parameters.

Example #9 Correct usage of default function arguments

<?php
function makeyogurt($container = "bowl", $flavour = "raspberry", $style = "Greek")
{
    return "Making a $container of $flavour $style yogurt.\n";
}

echo makeyogurt(style: "natural");
?>

The above example will output:

Making a bowl of raspberry natural yogurt.

As of PHP 8.0.0, declaring mandatory arguments after optional arguments is deprecated. This can generally be resolved by dropping the default value, since it will never be used. 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 #10 Declaring optional arguments after mandatory arguments

<?php
 function foo($a = [], $b) {} // Default not used; deprecated as of PHP 8.0.0
 function foo($a, $b) {}      // Functionally equivalent, no deprecation notice

 function bar(A $a = null, $b) {} // Still allowed; $a is required but nullable
 function bar(?A $a, $b) {}       // Recommended
 ?>

Note: As of PHP 7.1.0, omitting a parameter which does not specify a default throws an ArgumentCountError; in previous versions it raised a Warning.

Note: Arguments that are passed by reference may have a default value.

Variable-length argument lists

PHP has support for variable-length argument lists in user-defined functions by using the ... token.

Argument lists may include the ... token to denote that the function accepts a variable number of arguments. The arguments will be passed into the given variable as an array:

Example #11 Using ... to access variable arguments

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

echo sum(1, 2, 3, 4);
?>

The above example will output:

... can also be used when calling functions to unpack an array or Traversable variable or literal into the argument list:

Example #12 Using ... to provide arguments

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

echo add(...[1, 2])."\n";

$a = [1, 2];
echo add(...$a);
?>

The above example will output:

3
3

You may specify normal positional arguments before the ... token. In this case, only the trailing arguments that don't match a positional argument will be added to the array generated by ....

It is also possible to add a type declaration before the ... token. If this is present, then all arguments captured by ... must match that parameter type.

Example #13 Type declared variable arguments

<?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);
?>

The above example will output:

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

Finally, variable arguments can also be passed by reference by prefixing the ... with an ampersand (&).

Named Arguments

PHP 8.0.0 introduced named arguments as an extension of the existing positional parameters. Named arguments allow passing arguments to a function based on the parameter name, rather than the parameter position. This makes the meaning of the argument self-documenting, makes the arguments order-independent and allows skipping default values arbitrarily.

Named arguments are passed by prefixing the value with the parameter name followed by a colon. Using reserved keywords as parameter names is allowed. The parameter name must be an identifier, specifying dynamically is not allowed.

Example #14 Named argument syntax

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

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

Example #15 Positional arguments versus named arguments

<?php
// Using positional arguments:
array_fill(0, 100, 50);

// Using named arguments:
array_fill(start_index: 0, count: 100, value: 50);
?>

The order in which the named arguments are passed does not matter.

Example #16 Same example as above with a different order of parameters

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

Named arguments can be combined with positional arguments. In this case, the named arguments must come after the positional arguments. It is also possible to specify only some of the optional arguments of a function, regardless of their order.

Example #17 Combining named arguments with positional arguments

<?php
htmlspecialchars($string, double_encode: false);
// Same as
htmlspecialchars($string, ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML401, 'UTF-8', false);
?>

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

Example #18 Error thrown 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
?>

As of PHP 8.1.0, it is possible to use named arguments after unpacking the arguments. A named argument must not override an already unpacked argument.

Example #19 Use named arguments after unpacking

<?php
function foo($a, $b, $c = 3, $d = 4) {
  return $a + $b + $c + $d;
}

var_dump(foo(...[1, 2], d: 40)); // 46
var_dump(foo(...['b' => 2, 'a' => 1], d: 40)); // 46

var_dump(foo(...[1, 2], b: 20)); // Fatal error. Named parameter $b overwrites previous argument
?>

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User Contributed Notes 14 notes

down

121

php at richardneill dot org ¶

9 years ago


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".
*/
?>

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Simmo at 9000 dot 000 ¶

2 years ago


For anyone just getting started with php or searching, for an understanding, on what this page describes as a "... token" in Variable-length arguments:
https://www.php.net/manual/en/functions.arguments.php#functions.variable-arg-list
<?php

  func($a, ...$b) 

?>
The 3 dots, or elipsis, or "...", or dot dot dot is sometimes called the "spread operator" in other languages.

As this is only used in function arguments, it is probably not technically an true operator in PHP.  (As of 8.1 at least?).

(With having an difficult to search for name like "... token", I hope this note helps someone).

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LilyWhite ¶

3 years ago


It is worth noting that you can use functions as function arguments

<?php
function run($op, $a, $b) {
  return $op($a, $b);
}

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

$mul = function($a, $b) {
  return $a * $b;
};

echo run($add, 1, 2), "\n";
echo run($mul, 1, 2);
?>

Output:
3
2

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tianyiw at vip dot qq dot com ¶

2 years ago


<?php
/**
 * Create an array using Named Parameters.
 *
 * @param mixed ...$values
 * @return array
 */
function arr(mixed ...$values): array
{
    return $values;
}

$arr = arr(
    name: 'php',
    mobile: 123456,
);

var_dump($arr);
// array(2) {
//   ["name"]=>
//   string(3) "php"
//   ["mobile"]=>
//   int(123456)
// }

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gabriel at figdice dot org ¶

8 years ago


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
?>

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Hayley Watson ¶

7 years ago


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.

down

boan dot web at outlook dot com ¶

6 years ago


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
?>

down

Luna ¶

2 years ago


When using named arguments and adding default values only to some of the arguments, the arguments with default values must be specified at the end or otherwise PHP throws an error:

<?php

function test1($a, $c, $b = 2)
{
    return $a + $b + $c;
}

function test2($a, $b = 2, $c)
{
    return $a + $b + $c;
}

echo test1(a: 1, c: 3)."\n"; // Works
echo test2(a: 1, c: 3)."\n"; // ArgumentCountError: Argument #2 ($b) not passed

?>

I assume that this happens because internally PHP rewrites the calls to something like test1(1, 3) and test2(1, , 3). The first call is valid, but the second obviously isn't.

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Sergio Santana: ssantana at tlaloc dot imta dot mx ¶

19 years ago


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.

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jcaplan at bogus dot amazon dot com ¶

18 years ago


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

?>

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catman at esteticas dot se ¶

8 years ago


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

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Hayley Watson ¶

7 years ago


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.

down

info at keraweb dot nl ¶

7 years ago


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"

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John ¶

18 years ago


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)

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