当您希望简单地更改原始变量并将其返回到相同的变量名并赋予其新值时，可以将此用于函数。

function add(&$var){ // The & is before the argument $var
   $var++;
}
$a = 1;
$b = 10;
add($a);
echo "a is $a,";
add($b);
echo " a is $a, and b is $b"; // Note: $a and $b are NOT referenced

关于如何将对象传递给函数，你仍然需要理解，没有“&”，你传递给函数的是一个对象句柄，对象句柄仍然是通过值传递的，它包含一个指针的值。但你不能改变这个指针，直到你通过引用传递它使用“&”

<?php
        class Example 
        {
            public $value;
         
        }
        
        function test1($x) 
        {
             //let's say $x is 0x34313131
             $x->value = 1;  //will reflect outsite of this function
                             //php use pointer 0x34313131 and search for the 
                             //address of 'value' and change it to 1

        }
        
        function test2($x) 
        {
             //$x is 0x34313131
             $x = new Example;
             //now $x is 0x88888888
             //this will NOT reflect outside of this function 
             //you need to rewrite it as "test2(&$x)"
             $x->value = 1000; //this is 1000 JUST inside this function
                 
        
        }
         
     $example = new Example;
    
     $example->value = 0;
    
     test1($example); // $example->value changed to  1
    
     test2($example); // $example did NOT changed to a new object 
                      // $example->value is still 1
     
 ?>

两种方法都可以。

在前面放一个“&”符号，你传递的变量就变成了它的原点，也就是说，你可以通过引用传递，而不是复制它。

so

    $fred = 5;
    $larry = & $fred;
    $larry = 8;
    echo $fred;//this will output 8, as larry and fred are now the same reference.

http://www.php.net/manual/en/migration5.oop.php

In PHP 5 there is a new Object Model. PHP's handling of objects has been completely rewritten, allowing for better performance and more features. In previous versions of PHP, objects were handled like primitive types (for instance integers and strings). The drawback of this method was that semantically the whole object was copied when a variable was assigned, or passed as a parameter to a method. In the new approach, objects are referenced by handle, and not by value (one can think of a handle as an object's identifier).

实际上这两种方法都是有效的，但这取决于你的需求。通过引用传递值通常会使脚本变慢。因此，考虑到执行时间，最好按值传递变量。此外，当按值传递变量时，代码流更加一致。

可以通过引用将变量传递给函数。这个函数将能够修改原始变量。

你可以在函数定义中通过引用来定义段落:

<?php
function changeValue(&$var)
{
    $var++;
}

$result=5;
changeValue($result);

echo $result; // $result is 6 here
?>