Usage of referenced symbols (&) in php

Php references add symbols before variables, functions, and objects. in PHP, references mean that different names access the same variable content, the following describes how to use PHP to reference a pointer different from that in C. the pointer in C language stores the reference of the address variable content stored in the memory.
PHP reference allows you to use two variables to point to the same content
The code is as follows:
$ A = "ABC ";
$ B = & $;
Echo $ a; // output here: ABC
Echo $ B; // output here: ABC
$ B = "EFG ";
Echo $ a; // Here, the value of $ a is changed to EFG, So EFG is output.
Echo $ B; // output EFG here

Function address transfer call
I will not talk much about the address transfer call. the following code is provided directly.
The code is as follows:
Function test (& $)
{
$ A = $ a + 100;
}
$ B = 1;
Echo $ B; // output 1
Test ($ B); // here, $ B actually transmits the memory address of the variable content of $ B to the function, you can change the value of $ B by changing the value of $ a in the function.
Echo"
";
Echo $ B; // output 101

Note that test (1); will cause an error.
Function Reference return
First look at the code
The code is as follows:
Function & test ()
{
Static $ B = 0; // declare a static variable
$ B = $ B + 1;
Echo $ B;
Return $ B;
}

$ A = test (); // This statement outputs the value of $ B as 1.
$ A = 5;
$ A = test (); // This statement outputs the value of $ B to 2.

$ A = & test (); // This statement outputs the value of $ B to 3.
$ A = 5;
$ A = test (); // This statement outputs a value of 6 for $ B.

The following explains:
In this way, $ a = test (); is not actually returned by the function reference, which is no different from the normal function call. The reason is: this is the PHP rule.
PHP requires that $ a = & test (); is used to obtain the function reference and return.
As for what is reference return (in the PHP Manual, reference return is used when you want to use a function to find the variable on which the reference should be bound .) I haven't understood this sentence for a long time.
The example above is as follows:
$ A = test () is used to call a function. it only assigns the value of the function to $ a. any change made to $ a does not affect $ B in the function.
But how to call a function through $ a = & test, the function is to direct the memory address of the $ B variable in return $ B to the same place as the memory address of the $ a variable.
That is, the equivalent effect ($ a = & B;) is generated. Therefore, changing the value of $ a also changes the value of $ B.
$ A = & test ();
$ A = 5;
Later, the value of $ B is changed to 5.
Static variables are used to help you understand the reference and return functions. In fact, function reference and return are mostly used in objects.
Object reference
The code is as follows:
Class {
Var $ abc = "ABC ";
}
$ B = new;
$ C = $ B;
Echo $ B-> abc; // output ABC here
Echo $ c-> abc; // output ABC here
$ B-> abc = "DEF ";
Echo $ c-> abc; // output DEF here

The above code is the running effect in PHP5
In PHP5, object replication is implemented through reference. In the above column, $ B = new a; $ c = $ B; is equivalent to $ B = new a; $ c = & $ B;
In PHP5, the object is called by reference by default, but sometimes you may want to create a copy of the object and expect that the change of the original object will not affect the copy. for this purpose, PHP defines a special method called _ clone.
Role of reference
If the program is large, there are many variables that reference the same object, and you want to manually clear the object after it is used up, I suggest using the & method, then clear it in the form of $ var = null. in other cases, use the default php5 method. in addition, we recommend that you use the "&" method for transferring large arrays in php5 to save memory space.
Cancel reference
When you unset a reference, you just disconnect the binding between the variable name and the variable content. This does not mean that the variable content is destroyed. For example:
The code is as follows:
$ A = 1;
$ B = & $;
Unset ($ );

Not unset $ B, just $.
Global Reference
When a variable is declared with global $ var, a reference to the global variable is actually created. That is to say, it is the same as doing so:
The code is as follows:
$ Var = & $ GLOBALS ["var"];

This means that, for example, unset $ var does not unset global variables.
$ This
In the method of an object, $ this is always a reference to the object that calls it.
// Next is an episode
In php, the address pointing (similar to pointer) function is not implemented by the user, but is implemented by the Zend core. in php, the reference uses the principle of "copy at Write, unless a write operation occurs, the variables or objects pointing to the same address will not be copied.
In layman's terms
1: if the following code exists:
The code is as follows:
$ A = "ABC ";
$ B = $;

In fact, both $ a and $ B point to the same memory address, not $ a and $ B occupy different memory 2: if the following code is added on the basis of the above code
The code is as follows:
$ A = "EFG ";

Because the memory data pointed to by $ a and $ B needs to be re-written, the Zend core automatically determines that $ B will generate a $ a data copy, apply for a new memory for storage