How to understand the '&' symbol added to PHP custom functions?

In PHP, the & symbol passes a reference to a variable rather than a copy. Reference means accessing the same variable content with different names. This is not like C's pointers, which are symbol table aliases. Note that in PHP, the variable name and the variable content are different, so the same content can have different names. The closest analogy is Unix's filenames and the files themselves - the variable names are the directory entries, and the variable contents are the files themselves. References can be thought of as tight connections in a Unix file system.
PHP’s references allow you to use two variables to point to the same content. Meaning, when you do:

<?php 
$a =&$b 
?>

The same syntax can be used in functions, which return references, and in the new
operator
(PHP 4.0.4 and later):

<?php 
$bar =& new fooclass(); 
$foo =& find_var ($bar); 
?>

error messages
in a constructor, such as with @new, this has no effect when using the &new statement. This is a limitation of the Zend engine and will cause a parsing error. The second thing a reference does is pass a variable by reference. This is accomplished by creating a local variable within the function and that variable references the same content in the calling scope. For example:

<?php 
function foo (&$var) { 
$var++; 
} 
$a=5; 
foo ($a); 
?>

What a reference is not
As mentioned before, a reference is not a pointer. This means that the following structure will not produce the effect you expect:

<?php 
function foo (&$var){ 
$var =& $GLOBALS["baz"]; 
} 
foo($bar); 
?>

You can pass a variable to a function by reference so that the function can modify the value of its parameter. The syntax is as follows:

<?php 
function foo (&$var) { 
$var++; 
} 
$a=5; 
foo ($a); 
// $a is 6 here 
?>

Variables, such as foo($a)
New statements, such as foo(new foobar())
References returned from functions, such as:

<?php 
function &bar() { 
$a = 5; 
return $a; 
} 
foo(bar()); 
?>

expression
cannot be passed by reference, and the result is undefined. For example, the following example of passing by reference is invalid:

<?php 
function bar(){ // Note the missing & 
$a = 5; 
return $a; 
} 
foo(bar()); 
foo($a = 5) // 表达式，不是变量 
foo(5) // 常量，不是变量 
?>

Reference return is used when you want to use a function to find which variable the reference should be bound to. When returning a reference, use this syntax:

<?php 
function &find_var ($param){ 
/* ...code... */ 
return $found_var; 
} 
$foo =& find_var ($bar); 
$foo->x = 2; 
?>

Unreference
When you unset a reference, you just break the binding between the variable name and the variable content. This does not mean that the variable contents are destroyed. For example:

<?php 
$a = 1; 
$b =& $a; 
unset ($a); 
?>

Reference positioning
Many PHP syntax structures are implemented through the reference mechanism, so everything above about reference binding also applies to these structures. Some constructs, such as pass-by-reference and return, have already been mentioned above. Other structures that use references are:
global reference
When declaring a variable with global $var, a reference to the global variable is actually established. That is the same as doing:

<?php 
$var =& $GLOBALS["var"]; 
?>

这意味着，例如，unset $var 不会 unset 全局变量。
$this
在一个对象的方法中，$this 永远是调用它的对象的引用。

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