ltc-exer4-variables3.md

Assignment by reference

By default in most assignments, PHP uses assignment by value. When a variable is assigned value from another variable, the latter's value is assigned to the former.

$breakfast = "eggs";
$lunch = $breakfast;		// Assignment by value

// Print meals. Outputs:
// eggs
// eggs
echo $breakfast . "\n";
echo $lunch . "\n";

$lunch = "crispy pata";

// Print meals. Outputs:
// eggs
// crispy pata
echo $breakfast . "\n";
echo $lunch . "\n";

Assignment by reference means assigning a reference instead of a value. This results in multiple variables behaving as one.

$breakfast = "eggs";
$lunch = &$breakfast;		// Assignment by reference

// Print meals. Outputs:
// eggs
// eggs
echo $breakfast . "\n";
echo $lunch . "\n";

$lunch = "crispy pata";

// Print meals. Outputs:
// crispy pata
// crispy pata
echo $breakfast . "\n";
echo $lunch . "\n";

To understand assignment by reference, it is necessary to have a mental image of how the computer stores data. Variables don't actually contain the data but rather they only hold the "address" of data in computer memory.

When the computer reads the instruction $lunch = $breakfast, it interprets it as "assign the value of $breakfast to $lunch". The computer does this by allocating a new cell in memory to the new variable $lunch then copying the value of $breakfast at cell A1 over to $lunch at cell A3.

When the value of $lunch was modified, this actually meant the value of the memory cell with address A3 was modified.

On the other hand, when the computer reads the instruction $lunch &= $breakfast, it actually reads it as "assign the address of $breakfast to $lunch". The computer then shares the address of $breakfast's memory cell to $lunch resulting in both of them pointing to the same memory cell.

This time, when the value of $lunch is modified, it now refers to the value of the memory cell with address A1. And since both $breakfast and $lunch use the same memory cell, both variables will change value when the value in the cell is modified.

Exercise

In this exercise, your task is to trace some codes and determine the output.

1. What does the following piece of code display?

   $a = "foo";
   $b = &$a;
   $b = "Foo";
   echo $a;

2. What does the following piece of code display?

   $a = "foo";
   $b = &$a;
   $b .= "bar";
   echo $a;

3. What does the following piece of code display?

   $a = "foo";
   $b = &$a;
   $b .= $a;
   echo $a;

4. What does the following piece of code display?

   $a = "foo";
   $b = "bar";
   
   $c = &$a;
   $d = $b;
   
   $a .= $d;
   $b .= $c;
   $a .= $b;
   
   echo $c;

5. What does the following piece of code display?

   $a = "foo";
   $b = &$a;
   $c = "bar";
   $b = &$c;
   
   echo $a;

Submission

• Submit a file with 2 columns:
  • Column1: Item number
  • Column2: Answer
• Filename must be exer4-variables3.php

Sample

1 Foo
2 100
3 Foobar
4 FizzBuzz
5 Hello world

---

Gist: https://gist.github.com/czarpino/913e36c4379eb04e5ba425a0e4215802

reference1.png

reference2.png

value1.png

value2.png