Question

我不确定究竟是什么类型的参数列表行为被调用，因此很难搜索更多信息。我将举一个例子：

function echoThis( $a=NULL, $b=NULL, $c=NULL )
{
    if( $a != NULL )
        echo "A is not NULL; it is: " . $a . "\n";
    if( $b != NULL )
        echo "B is not NULL; it is: " . $b . "\n";
    if( $c != NULL )
        echo "C is not NULL; it is: " . $c . "\n";

}

function echoThat( $a=NULL, $b=NULL, $c=NULL )
{
    if( $a != NULL )
        echo "A is not NULL; it is: " . $a . "\n";
    elseif( $b != NULL )
        echo "B is not NULL; it is: " . $b . "\n";
    elseif( $c != NULL )
        echo "C is not NULL; it is: " . $c . "\n";
    else
        echo "Huh?\n";

}

$a = "hello";
$b = "how are you";
$c = "goodbye";

echoThis( $a );
echoThis( $a, $b );
echoThis( $a, $b, $c );
echo "--------\n";
echoThat( $a );
echoThat( $a, $b );
echoThat( $a, $b, $c );
echo "--------\n";
echoThis( $a );
echoThis( $b, $a );
echoThis( $c, $b, $a );
echo "--------\n";
echoThat( $a );
echoThat( $b, $a );
echoThat( $c, $b, $a );
echo "--------\n";
echoThis( $a );
echoThis( $b );
echoThis( $c );
echo "--------\n";
echoThat( $a );
echoThat( $b );
echoThat( $c );

?>

因此，无论传递的变量的顺序如何，它都会填充内部范围的变量，就好像第一个是$ a和第二个$ b以及最后的$ c，但是如果只提供一个参数，它会以某种方式匹配变量命名为正确的内部范围变量。因此，如果只传递$ c，它会跳过$ a和$ b并知道将其分配给$ c。

A is not NULL; it is: hello  
A is not NULL; it is: hello  
B is not NULL; it is: how are you  
A is not NULL; it is: hello  
B is not NULL; it is: how are you  
C is not NULL; it is: goodbye  
--------  
A is not NULL; it is: hello  
A is not NULL; it is: hello  
A is not NULL; it is: hello  
--------  
A is not NULL; it is: hello  
A is not NULL; it is: how are you  
B is not NULL; it is: hello  
A is not NULL; it is: goodbye  
B is not NULL; it is: how are you  
C is not NULL; it is: hello  
--------  
A is not NULL; it is: hello  
A is not NULL; it is: how are you  
A is not NULL; it is: goodbye  
--------  
A is not NULL; it is: hello  
A is not NULL; it is: how are you  
A is not NULL; it is: goodbye  
--------  
A is not NULL; it is: hello  
A is not NULL; it is: how are you  
A is not NULL; it is: goodbye

这叫什么，以便我可以找到更多信息？（或者这被认为是一种不好的做法？对我来说最有趣的是中间的两个节产生不同的结果，最后两个都使用第一个if语句，即使变量中的值被正确映射（我认为）。

Answer 1

如果你有

function say($first_argument = null, $second_argument = null) {
     echo $first_argument;
     echo $second_argument;
}

那么在该函数之外调用变量并不重要。你可能有

$random_variable = "hello";
$other_variable = " world";

say($random_variable, $other_variable);

//outputs "hello world"

这与

相同

say("hello", " world");

如果您希望能够通过原来的名称来引用变量，那么您真的不能。你可以传入一个数组：

 function say($args) {
     extract($args);
     echo $a;
     echo $b;
 }

 $args = array('a' => 'hello', 'b' => ' world');
 say($args); //hello world

这将允许您通过数组中的键引用变量，在这种情况下它将匹配它们。例如：

$args = array('b' => ' world', 'a' => ' hello');
say($args); //hello world.

Answer 2

您所看到的内容取决于函数参数的顺序，而不是变量名称。如果您的函数定义是

function echoThis( $a=NULL, $b=NULL, $c=NULL ) {
   ...
}

您将其称为

echoThis($a);

然后你会看到你期望的行为。但是，如果你打电话

echoThis($b);

然后参数（在这种情况下为$b）将映射到函数的第一个参数，在执行函数时将其称为变量$a。要达到预期的效果，您必须致电

echoThis (null, $b)

这将有效地忽略第一个参数。

有关详细信息，请参阅php.net/manual/en/functions.arguments.php。

Answer 3

这是应该的：

--------
this-a
A is not NULL; it is: hello
--------
this-ab
A is not NULL; it is: hello
B is not NULL; it is: how are you
--------
this-abc
A is not NULL; it is: hello
B is not NULL; it is: how are you
C is not NULL; it is: goodbye
--------
that-a
A is not NULL; it is: hello
--------
that-ab
A is not NULL; it is: hello
--------
that-abc
A is not NULL; it is: hello
--------
this-a
A is not NULL; it is: hello
--------
this-ba
A is not NULL; it is: how are you
B is not NULL; it is: hello
--------
this-cba
A is not NULL; it is: goodbye
B is not NULL; it is: how are you
C is not NULL; it is: hello
--------
that-a
A is not NULL; it is: hello
--------
that-ba
A is not NULL; it is: how are you
--------
that-cba
A is not NULL; it is: goodbye
--------
this-a
A is not NULL; it is: hello
--------
this-b
A is not NULL; it is: how are you
--------
this-c
A is not NULL; it is: goodbye
--------
that-a
A is not NULL; it is: hello
--------
that-b
A is not NULL; it is: how are you
--------
that-c
A is not NULL; it is: goodbye

每个函数都被正确调用，并没有陌生感。传递的第一个参数始终为$a，第二个参数始终为$b，第三个参数始终为$c。

在this-cba A变得再见而不是你好。

在that-cba A变得再见而不是你好。

that-b A成为how are you。 $a不为空，它首先被回显，其余的则没有。在所有这些中（this和that）A不是空的。如果它根据名称将它们分配给适当的变量，您会看到B不为空或C不为空。

什么函数参数属性允许这种奇数重载？

3 个答案: