我不确定这种参数列表行为到底叫什么，所以很难搜索更多信息。我举个例子：
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( $c );
?>
因此，无论传递的变量的顺序如何，它都会填充内部作用域变量，就好像第一个是$a的，第二个是$b和最后一个$c，但如果只提供一个参数，它会以某种方式将变量名称与正确的内部作用域变量匹配。因此，如果只传递$c，它会跳过$a并$b，并知道将其分配给$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？对我来说最令人讨厌的是中间的两节产生不同的结果，最后两节都使用第一个 if 语句触发，即使变量中的值也被正确映射(我认为(。
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什么变量并不重要。 你可以有
$random_variable = "hello";
$other_variable = " world";
say($random_variable, $other_variable);
//outputs "hello world"
这与
say("hello", " world");
如果您希望能够通过变量最初的名称来引用变量，那么您真的不能。 你可以传入一个数组：
function say($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.
这是它应该工作：
--------
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，没有陌生感。 传递的第一个参数将始终$a，第二个参数将始终$b，第三个参数将始终$c。
在this-cba A变成再见而不是你好。
在that-cba A成为再见而不是你好。
在that-b A变得how are you. $a 不为 null，它首先被回显出来，其余的则不为空。 在所有这些最后的(this和that(中，A不是空的。 如果根据名称将它们分配给正确的变量，您将看到B不为空或C不为空。
您看到的内容取决于函数参数的顺序，而不是变量名称。如果函数定义为
function echoThis( $a=NULL, $b=NULL, $c=NULL ) {
...
}
你称之为
echoThis($a);
然后你会看到你期望的行为。但是，如果您要致电
echoThis($b);
然后参数(在本例中为 $b (将映射到函数的第一个参数，该参数在执行函数时被引用为变量$a。为了达到您期望的结果，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。