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Detailed explanation of PHP variable functions and function reference and dereference examples
Detailed explanation of PHP variable functions and function reference and dereference examples
变量函数
在 php 中是支持变量函数的，首先定义一些函数，再声明一个变量，使用变量来调用不同的函数，然后不断地重新给变量赋值。下面我们将通过一个实例来介绍变量函数的具体使用方法。在这个例子中首先定义3个函数，接着声明一个变量并赋值，通过变量函数来访问不同的3个函数，需要重新不断地给变量赋值。
其示例的代码如下所示：
<?php
function come(){
echo "进门了<br>";
}
function go($name="tom"){
echo "$name 出门了<br>";
}
function back($str){
echo "重新进门 $str<br>";
}
$func = "come"; //声明一个变量并赋值
$func(); //使用变量函数来调用函数come()
$func = "go"; //重新给变量赋值
$func("jane"); //使用变量函数来调用函数go()
$func = "back"; //重新给变量赋值
$func("jack"); //使用变量函数来调用函数back()
?>运行结果如下所示：
说明：可以看到，变量函数的调用时通过改变变量名来实现的，通过在变量名后面加上一对小括号，php将自动寻找与变量名相同的函数，并执行它。如果找不到对应的函数，系统将会报错。这项技术可以用于实现回调函数和函数表等等。
函数的引用
函数间参数的传递中的按引用传递的方式可以修改实参的内容。引用不仅可以用于普通变量、函数的参数，可也以用于函数本身。对函数的引用，就是对函数返回结果的引用。
下面我们将通过一个实例来说明，实现定义一个函数，这里需要在函数名前加上 “ & ” 符，接着，变量$str 将引用该函数，最后输出该变量$str, 实际上就是 $tmp的值。
实例代码如下所示：
<?php
function &test($tmp=0){ //定义一个函数，加 & 符号
return $tmp; //返回参数 $tmp
}
$str = &test("hello word"); //声明一个函数的引用$str
echo $str; //输出$str
?>输出结果为 ： hello word
注意：和参数传递不同，对函数的引用必须在两个地方都使用 “ & ” 符，用来说明返回的是一个引用。
取消引用
当在实际操作中不需要引用时，可以取消参数的引用传递方式的引用，只需要使用PHP函数即可，这里取消引用使用 unset() 函数，它只是断开了变量名和变量内容之间的绑定，而不是销毁变量内容。
下面通过一个实例来说明，首先声明一个变量和一个变量引用，输出引用后取消引用，再次调用引用和原变量。可以看到，取消引用后对原变量没有任何影响。
实例代码如下所示：
<?php $num = 123; $math = &$num; echo "\$math is: ".$math . "<br>"; unset($math); echo "\$math is: ".$math . "<br>"; echo "\$num is: " .$num; ?>
运行结果为：
$math is: 123 $math is: $num is: 123
以上就是php自定义函数的变量函数以及函数引用和取消引用的内容实例介绍，下一章我们开始学习php自定义函数变量的范围。
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