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。
Example #1 向函数传递数组
<?php
function takes_array($input)
{
echo "$input[0] + $input[1] = ", $input[0]+$input[1];
}
?>
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 #2 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 $str; // outputs 'This is a string, and something extra.'
?>
函数可以定义 C++ 风格的标量参数默认值，如下所示：
Example #3 在函数中使用默认参数
<?php
function makecoffee($type = "cappuccino")
{
return "Making a cup of $type.\n";
}
echo makecoffee();
echo makecoffee(null);
echo makecoffee("espresso");
?>
以上例程会输出：
Making a cup of cappuccino. Making a cup of . Making a cup of espresso.
PHP 还允许使用数组 array 和特殊类型 NULL 作为默认参数，例如：
Example #4 使用非标量类型作为默认参数
<?php
function makecoffee($types = array("cappuccino"), $coffeeMaker = NULL)
{
$device = is_null($coffeeMaker) ? "hands" : $coffeeMaker;
return "Making a cup of ".join(", ", $types)." with $device.\n";
}
echo makecoffee();
echo makecoffee(array("cappuccino", "lavazza"), "teapot");
?>
默认值必须是常量表达式，不能是诸如变量，类成员，或者函数调用等。
注意当使用默认参数时，任何默认参数必须放在任何非默认参数的右侧；否则，函数将不会按照预期的情况工作。考虑下面的代码片断：
Example #5 函数默认参数的不正确用法
<?php
function makeyogurt($type = "acidophilus", $flavour)
{
return "Making a bowl of $type $flavour.\n";
}
echo makeyogurt("raspberry"); // won't work as expected
?>
以上例程会输出：
Warning: Missing argument 2 in call to makeyogurt() in /usr/local/etc/httpd/htdocs/phptest/functest.html on line 41 Making a bowl of raspberry .
现在，比较上面的例子和这个例子：
Example #6 函数默认参数正确的用法
<?php
function makeyogurt($flavour, $type = "acidophilus")
{
return "Making a bowl of $type $flavour.\n";
}
echo makeyogurt("raspberry"); // works as expected
?>
以上例程会输出：
Making a bowl of acidophilus raspberry.
Note: 自 PHP 5 起，传引用的参数也可以有默认值。
PHP 在用户自定义函数中支持可变数量的参数列表。在 PHP 5.6 及以上的版本中，由 ... 语法实现；在 PHP 5.5 及更早版本中，使用函数 func_num_args()，func_get_arg()，和 func_get_args() 。
In PHP 5.6 and later, argument lists may include the ... token to denote that the function accepts a variable number of arguments. The arguments will be passed into the given variable as an array; for example:
Example #7 Using ... to access variable arguments
<?php
function sum(...$numbers) {
$acc = 0;
foreach ($numbers as $n) {
$acc += $n;
}
return $acc;
}
echo sum(1, 2, 3, 4);
?>
以上例程会输出：
10
You can also use ... when calling functions to unpack an array or Traversable variable or literal into the argument list:
Example #8 Using ... to provide arguments
<?php
function add($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
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 hint before the ... token. If this is present, then all arguments captured by ... must be objects of the hinted class.
Example #9 Type hinted 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);
?>
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
Finally, you may also pass variable arguments by reference by prefixing the ... with an ampersand (&).
No special syntax is required to note that a function is variadic; however access to the function's arguments must use func_num_args(), func_get_arg() and func_get_args().
The first example above would be implemented as follows in PHP 5.5 and earlier:
Example #10 Accessing variable arguments in PHP 5.5 and earlier
<?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