取决于版本，4是值，5是引用。

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.

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
     
 ?>

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).

根据PHP文档，它是按值计算的。

默认情况下，函数实参是按值传递的(这样，如果函数内实参的值改变了，它不会在函数外被改变)。为了允许函数修改其参数，它们必须通过引用传递。 要让函数的实参始终通过引用传递，请在函数定义中的实参名称前加上&。

<?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.'
?>

PHP变量按值赋值，按值传递给函数，当包含/表示对象时通过引用传递。可以使用“&”强制变量通过引用传递。

由值/引用示例赋值:

$var1 = "test";
$var2 = $var1;
$var2 = "new test";
$var3 = &$var2;
$var3 = "final test";

print ("var1: $var1, var2: $var2, var3: $var3);

输出:

Var1:测试，var2:最终测试，var3:最终测试

按值/引用示例传递:

$var1 = "foo";
$var2 = "bar";

changeThem($var1, $var2);

print "var1: $var1, var2: $var2";

function changeThem($var1, &$var2){
    $var1 = "FOO";
    $var2 = "BAR";
}

输出:

foo, var2 BAR

引用示例传递的对象变量:

class Foo{
    public $var1;

    function __construct(){
        $this->var1 = "foo";
    }

    public function printFoo(){
        print $this->var1;
    }
}


$foo = new Foo();

changeFoo($foo);

$foo->printFoo();

function changeFoo($foo){
    $foo->var1 = "FOO";
}

输出:

喷火

(最后一个例子可能更好。)